Check Engine LightTrouble Code - P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Welcome to, the place dedicated to that DTC (diagnostic trouble code) P0420. The P0420 code is a very popular one and that's why there's a site dedicated to that specific trouble code. If your car's check engine light is on, get that code read in case it's a P0420.

A P0420 code is a trouble code on OBD-II equipped vehicles. All vehicles from 1996-newer sold in North America support this generic powertrain code. This code is a pretty common one and this site is here to help the average do-it-yourself (DIY) car owner (like you) troubleshoot, diagnose, and fix their vehicle themselves when presented with a P0420 code, saving money.

If your car has multiple trouble codes (DTC's), it's always a good idea to fix them in the order they come up on the code reader. For example, if you have these codes - P0300, P0420, P0171, then you're going to want to tackle the P0300 Random Misfire code first (resolve it), and then tackle the P0420 code, etc. The reason for that is that the first code could be causing further DTCs to be set.

If you've come to this site you most likely already know that you have a P0420 code. To start, we strongly recommend you seek the services of a professional mechanic/technician if you have any doubts as to whether you can perform the repair! Also, be sure to follow proper diagnostic procedures to determine whether a part has failed. If you simply change parts hoping it will solve the problem, you're really just guessing and wasting money. This site is here for information purposes only and is meant to help you learn more about the dtc and decide if you can tackle the repair yourself. It is not intended as car repair advice.

OK, so what does a P0420 code mean anyway?

As mentioned above, the code's description is Catalyst System Efficiency Below Threshold (Bank 1). OK, now tell me that again in plain English please.... let's break it down: the catalyst system being referred to is your three-way catalytic converter; so the catalytic converter is not working properly (i.e. it's not efficient as it should be).

Bank 1 refers to the side of the engine which contains cylinder #1. On inline engines such as 4 & 5 cylinder engines, there is only one bank. So, to really simply things let's just say that the efficiency of the catalytic converter and/or exhaust pipe and/or O2 sensor is not so great. FYI this code is identical to P0430 however that code is for bank 2. You may even find you have both P0420 and P0430 trouble codes at the same time.

There are no symptoms so why should I fix it?

Great question! A P0420 code will cause no symptoms aside from the MIL (malfunction indicator light) will illuminate. The reality is there is still something wrong and your car needs attention in order to be in excellent running condition. A poorly maintained car will run less efficiently, burn more gas, and cost you more money in the long run.

Plus, if your check engine light is on for a P0420 code and you decide not to fix it, another more serious code could be triggered and you'd never know. The MIL lights up whether you have one code or ten! While I have your attention, don't forget to do oil changes as the owner's manual schedules and keep those tires inflated to the proper PSI.

So what is causing my P0420 code?

There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc.

OK, so I want to diagnose things, what do I do next?

The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those symptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc.

If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilloscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.

A few additional DIY friendly tips include:

On some vehicles such as some Subaru models an ECM reflash will fix the problem. As you can see a P0420 can be caused by many things, so if you have any doubts as to whether you can fix the problem yourself, please seek the advice of a professional technician. Good luck!

If after some diagnosis you find you do need to replace the converter, please consider buying one using this link from Amazon as we get a small commission if you buy anything after clicking the link. Thanks!

Other P0420 reading

Here are a few links to some other sites with great P0420 articles:

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User Comments

Please browse the comments below for tips and tricks from other vehicle owners and technicians.

07 civic p0420/p0420pd
I recently installed a header to my 2007 civic lx, ran fine for a week and now the engine light is on. No performance failure at all. I checked the exhaust leak and its fine what may be the problem? thank you.
#1 - Christian Herrera - 05/18/2009 - 17:46
PO420 code
I have a 2002 Chevy Tahoe. My service engine light has come on and a mechanic hooked it up and said it was Bank 1 Cat. Converter. They did not do anything further than read the code. I took it to advance and the read the same code. I am getting different things told to me the one mech. want to replace the conv. and I have another saying they think it is something else & I need to have it looked at closer. They said they have never replaced a cat. converter where the vehicle is not acting up. Should I have it looked at further or just replace the cat. converter?
#2 - Tessa - 05/21/2009 - 11:07
2002 saturn p0420 code
changed o2 sensor ,cool temp sensor,engine light is still on. what should i do next
#3 - scott - 06/09/2009 - 12:27
i don't know what part is this
this is for mitsubishi eclipse 1998 2.0 16 valves the code is p0420 Catalyst Efficiency Below Threshold (Bank 1) what i have to buy to fix my car?
#4 - miguel - 06/10/2009 - 21:38
i have a saab 95t 2000,
i have replaced the 2 02 sensors but i am still getting the same code, p0420.
i guess the CAT has give up.
#5 - neil - 06/14/2009 - 09:54
P0420 bank 2
I have an 03 trailblazer- scan says bank 2. Does that make the code more specific for my engine?
#6 - Aaron - 06/27/2009 - 15:18
downstream 02 sensor and cat
my knowlege about downstream o2 sensor is that it monitores how the cat operate , and if its reading like normal pre-cat sensor reading then we know the cat is going to be bad , it just tell us the cat is bad or healty, so i would like to know whether faulty downstream o2 sensor has effect on the integrity of the cat and how?
#7 - abdullbasit - 07/15/2009 - 14:58
I just replaced the cat on my 01 honda crv 2.0 now the check eng light is back on again what else could it be advanced auto said the oxe sencor. What do you think.
#8 - james weakley - 07/18/2009 - 19:42
po420 on 01 ACURA MDX
I just replace my oxygen sensor on my Acura MDX 3.5 liter v-tech engine, but the engine light i s back on i just wondering If its the catalytic converter is not working propoerly and needs to be replace too as well? I need some feedbacks and sugestions what will i do to fix the problem.
#9 - eugene m.baltazar - 07/25/2009 - 03:15
I have a 2005 Ford Taurus with only 54k miles on it. My check engine light came on and code P0300 came up and I was told I needed new spark plugs and wires. I had those replaced and the car ran terrible so I took it back. They informed me that they had crossed the coil wires and fixed it. The light was out and the car ran fine. The light came back on 3 days later. I took it back and they stated it was now a P0420 code which was the catalyst and I needed to take it to Ford because it was under warranty mandated by the State. I took it to Ford, paid for another diagnostic test and was told they could not check the catalyst until "a leak at the O ring" was fixed. I could not afford it so I parked the car and started saving money. Now I am being told again by another dealership that it sounds like a warranty issue. Is this true? I am getting very aggravated about the whole thing!
#10 - Carol - 08/25/2009 - 15:58
1998 BMW 318ti
My CEL came on throw'n the dreaded P0420. It was the Cat. and upstream O2 sensor, which I replaced (after mkt Cat.). 8K miles later, the CEL came back on with, you guessed it, P0420. I replaced the downstream O2 sensor and tested the MAF, which seems OK. No luck. Any ideas, please prey-tell!! I'm at my wits end here. Vacum leaks are unlikely. What could cause damage to a Cat.? Am I destroying another expensive Cat.? NO! EGR valve?
#11 - Mike B. - 09/03/2009 - 21:54
I have Honda civic Lx 1998 and yesterday the engine check light was on while driving. Got it checked and it i was told that it is P0420 code which suggests malfunction of Cat convertor or O2 sensors. As I read online that cat converter replacement is expensive i dont wanna go for that. Any suggestions what to do. As I have my state inspection expiring shortly does it has to be resolved before that.
#12 - sharma - 09/12/2009 - 15:55
i have 2001 saturn i have a light coming engin servic soon then i scan obd2 the coming p0420 cood now i have to change the 02 senser
#13 - jaspal - 09/18/2009 - 17:19
Just installed new header and high flow cat, now got an engine light on
Just installed new header and high flow cat on my 96 Honda Accord. Now I got an engine light on. Code is P0420. Will a high flow cat make this light come on? Also is a high flow cat bad?
#14 - Jason - 10/06/2009 - 21:54
2001 olds aurora
Will removing the resonator from my 2001 olds aurora turn my check engine light on.
#15 - edgar - 10/08/2009 - 01:28
how do i?
i have an 03 pontiac vibe with OBDII code P0420 coming up. i think i have an exhaust leak. how do i check for an exhaust leak?
#16 - paul - 10/25/2009 - 12:38
1996 pontiac grand am se!!!!!!!!
I have just recently chaged my o2 sensor and my check engine light just came on and I checked it with a scanner and the po420 code came up. So i was wondering if my cat. converter went or not. I also just had my exhaust snap just before the cat. converter and the shop i took it 2 cost me 54 bucks just for the guy 2 cut off the braket and weld a peice of pipe 2 it. will this cost me more 2 replace my cat. converter. could that be y my check engine light is on?
#17 - 1996 pontiac grand am se!!!!!! - 10/26/2009 - 09:56
98 civic p0420
i replaced the catalic converter on my 98 civic but never cleared the code do i need to do that or will it come of on its own
#18 - vitali - 10/28/2009 - 13:59
I have a good one for "ALL", and I consulted the professionals, which could not figure it out either. I had a P0420 code, so I replaced the Catalytic Converter with a cheap $60 universal, which had the wrong size pipe ends. So I went with the next best thing and the correct size pipes, $80, but it did not help, still had the Check Engine light. Then I tried the next best thing to that, $120, lasted 2 weeks, and still had the light. So I went to the next best thing, "Direct Fit" , no welds... $250, and "NO HELP".
I was told it had leaks in the system so I changed "ALL" gaskets 2-3 times. Then I was told it was the sensors, so with 140K miles, I changed them both... "NO HELP". I went to another exhaust expert, told him what I've done, and he said it was my "Fuel Trim"... which is factory set, so I do not do any thing with that. We cleaned the injectors, new filter, cleaned the MAP sensor and intake system... "NO HELP".
"FINALLY" I went to another exhaust expert expecting to replace the CAT one more time, and he did not want to take my money just then... so he sent me down the road to an Alley mechanic. I explained to him what I have done, and with out hesitation, he solved it... "California Emissions". That is all he said, and it dawned on me that he could be right. "NO" this vehicle does not have any stickers in the engine compartment which tell you that, so there was no way of knowing. $200 for a "California Emissions CAT", we installed it our selves, so that saved me some money, and it is running smooth and fine now.
Deal with that and good luck to "ALL"

#19 - RAUL MURILLO - 11/03/2009 - 12:39
I am going through the same problems as Raul Murillo.#19. I have a 1997 GMC Sierra. I have changed the cats, oxygen sensors-more than once-,and the PCM. Cleared codes and drove 35 miles and poof-P0420 comes up.
I am in NC and believe the vehicle does not have California Emissions. Is there anyway to diagnose thie/DIY?
#20 - Mark - 11/19/2009 - 10:00
1999 dodge ram 1500 4x4 / P0420 code
got the P0420 at the state emissions center.
read the diagnose things here.
chkd the front & rear sensor visually, found the front sensor wires very tight from a transmission rebuild. routed the wires diffrently. took both connections apart and pluged back in. the only other thing was the battery terminals needed cleaning badly. cleaned very well, replaced the terminals, started the truck & the chk eng light was off. WOW, thnks for the info saved me 89 to 500 dollars. o yeah had to reset the clock. thnks vry much
#21 - b pell - 12/16/2009 - 16:13
2001 toyota corolla
had PO420 code after checking car out everything looked good i put shell vpower
gas in and within half a tank engine light
went out and hasn't returned try it
it might save you money and headaches
#22 - sam - 01/08/2010 - 21:16
catalyseur de ford freestar 2006
bonjour,le code po420 viens d'apparraitre sur mon vehicule y a t'il un danger d'incendie
#23 - guy blondeau - 01/13/2010 - 15:14
I have a 2002 Grand Am Pontiac and I have change the oxigen sensor and I have change the old catalytic converter for a universal converter and the check engine light keep showing the code p0420. what can it be?
#24 - Albert - 01/19/2010 - 19:39
2002 Buick Rendezvous
PO420 Bank1 keeps repeating after code has been cleared. Any suggestions besides what has been said here already? Thanks.
#25 - Todd - 01/20/2010 - 13:37
PO420 code
I have a 2005 Dodge Ram SLT 4x4, 5.7 l with 53000 miles. The engine light came on so I had it checked out with the scanner and it gave me a PO420 code which is the Catalyst efficiency low-bank 1. I read the previous posts and since the truck is only 4 almost 5 years old I've visually checked the catalytic converter and looked for exhaust leaks. Not seeing any and I'm a little frustrated and don't want to get hosed by a dealership. I do smell the "rotten egg" smell occasionally but haven't noticed much else such as power loss or problems with driveability. I'm am hoping that maybe it is just a rear oxygen sensor setting off the code but want to make sure. Anyone out there with suggestions? Any direction would be greatly appreciated.
#26 - JAKE - 01/22/2010 - 18:54
2003 Honda CR-V
I have this code in my 03 CRV. It had 157K miles in it.

Last month it came on. Last week the dealer reset it for me and after driving 50 miles, the code comes on again.

I dont know what to do now. It cost 1K to get it fix at the dealership. My emission test is next month too.

Whats your advice?
#27 - Philip - 01/24/2010 - 18:00
PO420 ?
Code PO420 popped up the other day just wondering what to do any help would be appreciated...
#28 - 2000 BMW 540i - 01/28/2010 - 21:12
2009 Dodge Ram Sport Hemi
My truck is only 5 months old and just recently had a Flowmaster muffler installed at my local speed shop. Two days after the installation my engine light came on. I went to the Dodge dealer and they scanned it and the P0420 code came up. They checked for exhaust leaks which they did not find. So they cleared the code and said it possibly could come back again. Two days later the engine light came back on. What can i do to fix this problem? Thank you for your time.
#29 - Eric - 01/28/2010 - 22:06
I have 06 ridgeline 120,000 and got this code, what should I do?
#30 - Rich Murphy - 01/31/2010 - 20:51
Code p0420
I have a 2001 Acura MDX , I used to put 87 gas in it and now i put 91 gas . when it gets to half tank i fill it up. Just yesterday the check engine light popped up and i used the OBD code reader and it says p0420 . so now i want to know what might be wrong with it. please help.
#31 - Bob - 02/22/2010 - 23:50
code p0420
06 chevy 4cyl 38k miles mil light on in town,off hwy town engine temp 172,hwy 195.will change thermostat.watched temp and o2 sensor with scan tool under driving conditions.highway(192)bank 2 showed slow town(172) o2 faster change,matching bank1.this throws the code.
#32 - doc - 03/23/2010 - 12:07
p0420 -- code i'm getting with my 2003 subaru, foresster, please how do I fix it ???????????? I had the sensor replaced with another used sensor,i'm not getting the cel-- check engine light and I drove 300 miles. After 200 miles I had it retested and still coming up with 1 code, which is ...........a/f ratio I believe is the latest code please help
#33 - Patrick Mulligan - 03/26/2010 - 22:42
P0420 96 Toyota Tercel
96 Toyota Tercel.Got a PO420 code then it would go away and come on,later I got a Cylinder 1 misfire code,I changed my plugs and a bad wire and it seemed to resolve the cylinder 1 misfire code.No problem with how the how car runs at all lately,now I am getting the P0420 code again.Took it to a shop called Buckys,he removed the o2 sensor and said that by doing that he could tell my cat was not great somehow.But the kicker is it is a car from California so I need some better type of cat.I am still not 100% sold it is the cat.Anyone have any advice or anyway I can get this fixed cheap and legal?
#34 - Bill - 04/03/2010 - 17:40
Freakin dealer sold me a car with exhaust work already done. They reset the code with the hand-held computer and sold it to me. A dealer is supposed to sell you something that will pass inspection - they screwed me and gave me a guarantee that doesn't cover anything at all. They claim they put in new o2 sensors but who knows. Prolly due to cat back kit on the car. Need to figure this out. I just got a ticket for not inspected the other day. So it's costing me $50 - $150 every time I get pulled over now. I read on a racing forum you can make o2 sensor spacers, and/or use a resistor rig that you can make from radio shack parts to stop the computer from reacting to the non-issue.
#35 - MrSuds - 04/09/2010 - 13:41
P0420 code
I have a 2002 Infinity G20 133,000 miles My engine light just came on 2 weeks after installing a new spectra air filter not sure if this is related. did not see any vacuum leaks. could it be a the catalytic converter? The rear o2 censor was replaced by infinity at 40,440 miles. any suggestions ??
#36 - Dirk - 04/24/2010 - 09:28
P0420 code
I have a toyota corolla 2000. My engine light won't come off. I have had the catalytic converter change twice, both oxygen sensors replace and I don't know what else to do. Any suggestions?
#37 - Roz - 05/01/2010 - 19:48
P0420 Acura 00 integra LS B18 no v-tec
Same issues, replaced cat twice, rear o2 twice, tried upper engine cleaner(seafoam), light keeps coming back....was told fuel may be causing the problem due to more ethenol in the fuel, used lucas fuel additive and still same...might try replacing battery, spark plugs, wires,cap, thermostat..noticed small black film on old cat after replacing...possible oil leak or poor burning mixture in engine, but no colored smoke???? not sure what else to try...i use a 10w30 in my engine should i try something like a synthetic blend or 5w30???
#38 - Luke - 05/11/2010 - 12:24
2005 dodge ram 1500
hi i have the 5.7 auto 115000km (only gets supreme gas)and iam getting a rotten egg smell form the truck had a po420 come up abouth 4 months ago and cleared the code and havent seen it come back but in the last week truck seems lazy all the time and mostly stinks on long up hill grades
#39 - craig - 05/14/2010 - 17:18
P0420 1999 infinti G20
I just bought a used G20 with a P0420 service code. Can anyone tell me what I should replace 1st ?
#40 - Michael - 05/22/2010 - 19:51
P0420 code fixed
I'm the one with the Toyota Corrola and it is now fixed. My problem was that I badly needed a tune up alone with the Cat. converter. It has been one month and the light has not come back on. I'm convinced that it's fixed now. So try the tune up with the cat. converter.
#41 - Roz - 06/03/2010 - 14:51
I was getting the p0420 error.
I changed the pcv valve out and it fixed it.
#42 - eric - 06/05/2010 - 15:06
po420 jetta 01
I replaced MAF sensor and pre o2,after 120 miles the code come up,mechanic told me:replace the post o2,if problem still the same,replace the I have a lot of money...any suggestion????????
#43 - il capitano - 06/08/2010 - 19:06
CRV 2000 rpm sensor for 0300 and 0420 codes
The last 5 months i have being dealing with this problem.
The first symptom i had was a lazy L1 sensor.
Besides that i had 0300 codes which indicated misfiring in all the cylinders (not just a particular one). After changing MAP sensor cleaned the intake sys,changed plugs changed plug wires, changed spark plugs drive unit and cup i noticed that nothing would fix the problem.
Suddenly i thote about checking the emissions.
I noticed that instead of max 100 i had like 350 on idle speed which is way off limits.
Decided to change the rpm sensor and that helped disappearing the above mentioned problems. Then another problem came up with the 0420 code. It was obvious that the car was running for 6 months with wrong injection timing which eventually damaged my CAT. One simple check you people can do to see if the problem is on your cat is the thermometer check. Intake temperature on the CAT should be lower than 100 C than the L2 side temperature everything bellow 100 C means a ready to die CAT. To do that you need a thermometer that measures 400 C.
The symptoms i get now is that 0420 light comes in to view during the morning first start. if i run the car for 5 to 10 km to warm up the cat and reset the code than the problem disappears. Also if i fill up with Super unleaded carb which has additives and does a better burn with less emissions then the problem disappears.
One thing also that people have to have in mind is that L2 sensor corrects what L1 misses, so if the L2 is not working properly this most likely would fire up the Check engine light.
Be careful!! Some times bad carb mixture can cause intermittent 0420 code which is the side effect of the misfiring!!
#44 - PAPPY - 06/16/2010 - 01:23
pontiac firebird 5.7v8
These converters have extended factory warranty's and will also set po420 code.also other vehicles wth 5.7 are covered.check with a dealer.have your vin.
#45 - Henry's auto - 07/05/2010 - 09:47
2002 mazda 626, 4cyl failed emission with code, p0421 warm up catalst efficiency below threshold bank 1. Can you advise of the proper repair, I am in urgent need of a response, than you.
#46 - simeon f sealey - 07/25/2010 - 08:36
99 toyota tacoma p0420 code
I recently replaced the motor in this truck. The wiring harness on the new motor was short for reaching the 02 sensor in the rear of the cat. I sautered the wires longer to reach. Now the check engine light came on with code p0420. Can you splice wires leading to the 02 sensor? Or is it the 02 bad, or maybe the cat has gon bad? Please help
#47 - Josh Wright - 07/25/2010 - 10:55
I have a 2000 honda passport 2wd, 3.2L V6 with a P0420 Check Engine Lite. I've had it in the shop for 2 days-nothing. They can't figure it out. Both CAT's can't be bad!? New O2 sensors front/rear-nothing. Engine seams to chug when driving-maybe a speed sensor? HELP!!! Jim
#48 - Jim - 07/28/2010 - 18:19
Is it my o2 sensor?
I just replaced my crankshaft position sensor, o2 sensor, and camshaft position sensor. One day later CEL came back on with p0420.

I did the o2 sensor myself so I'm wondering if its likely that it means I just need to re-wire it or if it's really something else going wrong.

2001 Mitsubishi Eclipse 2.4L, only two noticeable symptoms are a slightly hard time changing gears and the engine sometimes overheats when idling and will go back to normal once I start driving again.
#49 - Matt - 08/03/2010 - 16:52
2002 dodge ram van just failed insp because of p0420 below threshold bank 1
light stays on no leaks in cat or exhaust it looks good. Any advise?
Thanks Ed
#50 - Edward Nugent - 08/04/2010 - 12:45
P0420 code fix if all else fails
I had the same code P0420 bank 1 efficiency on a 2005 ram 1500 hemi. I had a custom cat back exhaust installed the code came on. I replaced the down stream O2 sensor. did not help. I replaced the cat, did not help. I replaced all gaskets, did not help. I then got frustrated and running out of patience. I searched around for possible cures. I found O2 spacers, BINGO no more code. Because of the increased flow the sensor could not react correctly. The spacer pull the sensor probe out of direct exhaust flow.

See if this helps.

Good luck,
#51 - Ed Williams - 08/05/2010 - 20:23
P0420 and Air Intake Hose
#52 - Bruce Vogel - 08/12/2010 - 11:37
Will this code alone stop Car from running
#53 - MYLES - 08/22/2010 - 22:18
420 code
I have a 2001 dodge durango with this code. Any help would be appreciated. Thanks MIKE
#54 - Mike Dunning - 08/28/2010 - 22:50
P0420 Toyota Carrolla
My check engine light came on so I brought it to the dealer and they ran a test that indicated I have a valve that's not working properly? Failed the emissions test of course so now I need to have it fixed. An emissions you know what they are talking about? I'm not seeing anything online about this valve with the P0420 trouble code and would like to have this fixed myself ( a friend if possible anyway). Thank you for your time and posting this site!

#55 - Amy - 09/29/2010 - 14:29
Please P0420

air leak in exhaust before rear HO2S
Fuel system fault
Faulty catalytic converter
My check engine light came on
Title number 86469416


#56 - ALI - 10/09/2010 - 23:10
I have a Camry 2004 with 127K miles, I brough to Firestone to have it tune up + fuel cleaning system + transmision system services and 4 wheels aligment done but next day the engine light come on with P0420 code when I brought back to Firestone
but they just reset then it come back on 2 days later.
#57 - Dinh Nguyen - 10/13/2010 - 21:01
How can i fix my catalyst system in my Subaru 2004
Kindly advice,
Can i remove the catalyst box away and make my car work without it...
Appreciate ur efforts,
#58 - Mina - 10/15/2010 - 23:25
I have a 2002 grand am V6 that has the P0420 code. I had the catalytic converter replaced in july of 2009 and the front O2 sensor replaced a few months ago. I have not replaced the rear O2 sensor. I have read that having an after- market cat converter installed can cause that code to pop up even thought it is running efficiently. Is the rear O2 sensor most likely my problem?
#59 - Kelly - 10/16/2010 - 14:26
Please P0420
Catalyst System Efficiency Below Threshold (Bank 1)

1. air leak in exhaust before rear HO2S. (heated oxygen sensor) BANK 1.
2. AF sensor error.
3. Fule sensor fault.
4. Faulty Catalytic converter.

I bought this toyota corolla 2003 with 98000 millage today, as it is from dealer. Have the engine lights on and giving the above error codes. Please guild us what we have to repair and it would require how much cost for parts. I
#60 - Rajnish Gupta - 10/17/2010 - 17:18
Hello.. did you get your answer..?
Hey.. for all those you guys asking all these questions.. Did any of you ever get any answer.. ?

Just see lots of questions here.. but none of the answer.. just curious.
#61 - Mike Jay - 10/20/2010 - 09:06

Ok, long story short, I have an Audi a4 Quattro, and after the first time my ignition coil went out, I for some oddly reason had my Catalytic converter changed out, and I noticed the p0420 code as soon aas all that happened,. Well converter changed, and 5 ignition coils later, and the contemplation of buying a new o2 sensor, I realized there was oil in my ignition coil pack every time they burnt out. SO I would reset my coed, and pop in new coils, and 20 minutes later the p0420 would come back. So I finally changed out my valve cover gasket, with a valve cover gasket kit from orieily's (19 bucks) cleaned out the coils, and spark plugs ports. Got some new plugs, and it has been two days, and not a p0420 code. I think that may have actually helped.. So before you waste all that money try the valve cover kit first clean out your ignition coils, or spark plug wires, and clear error code and hopefully that helps.. have a good one guys.
#62 - Fernando Sanchez - 10/25/2010 - 02:22
I have a 1998 Kia Sportage I'm working and I have 2 codes coming up can you help me with them the cordes are P0717 & P01614 The engine will run but try to drive it will stop running Thanks
#63 - Fred - 11/01/2010 - 22:13
subaru forester 2001
code p0420
how can i fix this problem?
#64 - jose - 11/03/2010 - 00:40
I just had a check engine and VSC light come up on my 98 Lexus GS300 yesterday. So i took it to autozone to get them to tell me what was causing it. once the Autozone tech hooked the reader to my car, it came up with code P0420 and P0306. So I came home and did a bit of research online and found a few basic items to check on my car that was easy to do. Out of the possible causes, I proceeded to check my radiator for coolant level. To my surprise, both my radiator and reservoir tank had no water and was empty. So i proceeded with adding some coolant/water to my radiator and reservoir tank. I then proceeded to starting up my vehicle and sure enough the lights were GONE!!! hooray. So i got lucky and didnt have any major repairs that was needed, but I will still be taking my car to a mechanic to see why i burnt through all that water - especially after replacing my radiator 6 months ago. But anyways, back to the subject at hand - So unlike what the Autozone was telling me that my catalyic converter may be bad, all i had to do was check the basics, and I was able to fix my own issue.
#65 - Vinny - 11/03/2010 - 21:58
big ass hole
i thank the one made this he is an asshole i spent lots of mony finaly fukin check enging apper shut
#66 - aboyazeed - 11/11/2010 - 06:22
P0420 98 Civic DX
I have a 98 Civic DX with 175,000 miles on it. The P0420 code just popped on and have read in many places that the catalytic converter is the probable cause. However i did see somewhere that it could possibly be fuel additives. I recently put one of those STP fuel injector cleaners in my last full tank of gas. Is this a probable cause and if so should i just go through a couple tanks of gas to see if it clears up?
#67 - Mr Plok - 11/19/2010 - 18:50
p0420 cause expensive engine damage
I just bought a lexus is300 and I popped up a p0420 code now a day after a exhaust valve dropped into the combustion chamber causing expensive engine damage.
#68 - lexus is300 - 11/21/2010 - 09:39
P0420 vs over 10% Ethanol gasoline

I've been reading A LOT on this P0420 problem and found out a few good thing you might like to know!!!

I lot of people have the same problem that we do, some changed everything related to emission system. Cats, o2 sensors, MAF, MAP, Temperature Sensor and still had the light coming on!!

On thing that not a lot a people think of checking is the Ethanol rating in the gasoline they use!! Toyota tacoma are set to use Gas with less then 10% ethanol... So what i did is I empty my tank and filled it back with ethanol Free Gasoline, unplug the battery to erase the codes and Guess what, no more light...

The problem with ethanol is that it doesn't burn like regular gas at all and our O2 sensors do not reconised the exhaust gas so your check engine light turn on!!!

I was using Gasoline that had 12% Ethanol and didn't know it!!

Try that and let me know what happens

#69 - andre - 11/27/2010 - 11:10
Holy crap!
So many people needing help and so few answering! guys, here is the thing, this code is for your CAT OR PRECAT(if applicable) OR BOTH. Most cats do not go bad by themselves, something else is usually the cause. Ie: bad injector, bad spark plug, bad wires or coil pac, etc. Something that is causing gasoline to enter the CAT, without completely burning the fuel, will foul it. I am willing to bet there were or are other codes with your P0420. If you changed your CAT but don't fix the other problem(s), you have just ruined another CAT. Bad O2 sensors, will put out a sensor code. So start looking at what caused the cat to go bad FIRST. Changing to a universal CAT, does not mean it works for your vehicle, regardless of the term "universal". Hope this helps some of you.
#70 - Venturo - 11/27/2010 - 14:48
to resolve the p0420 you can install and 02 and i will explaneand i have the part to repair,less the $20.00 bucks.
#71 - john - 12/01/2010 - 21:56
sorry,left out the email address for the's ""i will explain how the bung works and why the p0420 code sets.
#72 - john - 12/01/2010 - 21:59
old shoe
I will start off saying i own a 2001 firebird with 102,000 on it about a year ago i noticed a check engine light on the dash, po420 so i started looking on the internet for possible problems,changed the cat twice all o2 sensors then the complete exaust system,plugs,wires new exaust manifold gaskets, resealed the intake manifold,and guess what the po420 code was still there,i kept coming back to the internet hoping someone else had the same problem with no avail,so i started back tracking and one thing i noticed while changing the exaust pipes was there was little oil stains in the left exaust manifold, i also noticed when i would drive it then park it and then restart it she would blow some smoke out the exaust so i figured the valve seals were bad so i took it apart and found out the number three exaust valve seal had split open at the top and was leaking oil into cyclinder so i changed them out to,guess what when i started the motor the check engine light was out, just to double check it i put my scan tool on it , it said no dtcs in the system and you know whats funny i never saw anything on the internet about exaust valve seals being a possible problem to check for so i am adding this to the list as things to check for,i wish i had all that money back for the things i changed out,sometimes life is a bitch.
#74 - john - 12/03/2010 - 20:43
well, i've read more and convinced there's more problems than answers on here but i'm a novice when it comes to internet. ?;why would the one respondant say,"the downstream o2 could affect the upstream[bank one ]o2 cause insufficiency",when downstream o2 monitors results of bank 1&2 at the same time?please explain.the respondant with the firebird though got my attn.with the valve seals,though my jimmy's not smoking at start up or any other's just JIMMY P0420.i welcome your input!
#75 - leevus - 12/04/2010 - 20:49
responce to dependant
Well one thing i learned about the obd 2 system is that it will point you in the right direction, the po 420 code is for the left bank or side of the motor looking from the drivers seat in a rear wheel drive car,the computer is telling you through the o2 sensor on the left bank or left side of the motor that their is a problem in that area, in my case it was an oil seal on the left bank or side,the exaust valve on number 3 on my 3.8 motor was leaking oil into the left side exaust manifold contaminating the fuel mixture and setting a code.As the wise indian once said, look for signs,i hope this helps
#76 - old shoe - 12/05/2010 - 14:37
dependant 2
Things you might want to check,ok you know there is a problem on the left side, the first thing i would do is pull the plugs on the left side(bank i) and see how they look,things that it could be are a bad wire,bad plug,leaking intake manifold on that side,leaking exaust manifold on that side or like in my case bad exaust valve seal or to much blowby in a cyclinder on that side, whitch would show on one of the plugs looking oily than the rest on that side but you said you didn't see any smoke so we will rule that out,so plugs would be a good place to start.many times the intake manifold will leak and suck air, then the computer will try to compensate for that cyclinder and give it more fuel thus setting a code for that side with extra fuel that it doesn't need,the extra fuel would show up on the plug looking rich or dark,a bad wire or plug will show the same. to check for a manifold leak spray some carb cleaner on the intake runners on that side and see if the motor speeds up,if so you found your problem,if not keep looking,hope this helps
#77 - old shoe - 12/05/2010 - 17:25
what does a p2097 code on an 05 subaru impreza mean and how is it fixed?
#78 - lyr1 - 12/12/2010 - 19:48
420 problem..universal cat cut in...drove 1200 miles, no problem....I know need an inspection and the mil went on...420 code...had it reset and drove 70 miles went on again..???? I need help..they want to put in a new y pipe to the tune of 2k..please help 2002 gmc safari
#79 - Michael Bader - 12/15/2010 - 09:55
my husband has a custom exhaust on his '04 Ram 1500 4x4 5.7 liter Hemi. check engine light came on & it was diagnosed with a PO420 problem. he put on a new O2 censor but still has same problem. he was told that he may have to cut in another catalitic converter & he already has 2. trying to find another option.
#80 - Jen - 12/19/2010 - 17:09
1999 honda crv got po 420 code and the light went out on it`s own works on start up what would make the come on and go off without repair or resetting it
#81 - rick hardin - 12/27/2010 - 15:36
STP Fuel Injector Cleaner
I also got a P0420 CEL after putting STP Fuel Injector Cleaner in my tank.

My mechanic said sometimes people will get this readout in cold weather, so he put the light out.

It's only been a couple days, so I don't have any long-term results to report.

But I want to thank everyone who has posted some of their good results here. I have learned quite a lot today by reading these comments.
#82 - Darlene - 12/28/2010 - 11:20
tengo una troca nissan frontier xe 2001 tiene elcodigo p0420 pero eso tentra quever que el motor este aselerado o masvien las revulusiones delmotor RPM 1,200 4
#83 - nestor - 12/30/2010 - 02:06
Check leaks first
Check for Exhaust leak first, specialy close to O2 sensors... I had a very small leak on the second flange of # 2 cat and it triggered the p0420 code... fixed it and light came off!!

If you have a very small leak and you are running at a constant speed, it will pull air from the hole instread and fresh air will mess up the o2 sensor reading!!
#84 - Andre - 01/04/2011 - 13:10
2005 Altima 2.5
First tried new spark plugs at 105K , but same stuck SES light with P0420 code. After putting Fuel Injector Cleaner in my tank and driving another tank of gas, the light went out. ...Thanks for the tip Darlene.
#85 - Tedd B - 01/06/2011 - 22:36
I have a 2006 Chevrolet Malibu 2.2 engine, that the service engine light came on. Before it remained on I was experiencing loss of power when going over overpasses and my RPM's would increase but not speed. I had them run a scan on the car and it gave me PO 420 code, do you think it is the O2 sensor and which one would more than likely be the culprit?Your help is greatly appreciated.
#86 - Dale - 01/11/2011 - 16:09
2002 pontiac sunfire code p0420 #10
I payed 500$ to fix up some misfiring cylinders in my 02 pontiac sunfire. Ever since my ceck engine light was on and i checked it and got this exact code P0420 #10 and it said this 'Catalytic system efficiency threshhold below bank 1' what could this mean and how much would it cost for me to fix? Some help would be appreciated please!
#87 - Superlucky - 01/12/2011 - 16:26
Problem solved
The dreaded p420 code
Long story short 2001 jeep wrangler 160000 miles
I talked to a number if dealers/mechanics/autostores
Most said cat is bad some shops wanted to replace 02 sensors (I have 4)
Prices ranging from $800-$1500 plus. Thanks to the Internet one forum guy was like intake leak, check your manifold for cracks and or bad gasket, but I did and I did not appear to have a leak, guy responded do it anyways I'll put money on it ( not really gonna happen right?) anyways $20 in parts ( intake gasket ) and borrowed neighbors torque wrench. It too me a rookie 3 hrs. I reset the code. It's been 2 months and no CEL. And my gas mileage shot up immensely. Holo this helps.
#88 - Ken - 01/13/2011 - 09:40
98 2.0 Contour 150k. Drained dead on road due to alt light on. Replaced brushes charges great. While I was working on it I replaced leaking valve cover gasket, replaced plugs with Motorcraft plugs, replaced temp sensor, changed oil, and dumped into gas tank 2 bottles of water remover. (each bottle treats 10 gal, I had maybe 8 gal in tank). Car ran great for 40 miles. Traveled on snowy roads today. CEL came on today. code P0420. Wondering if water remover @ 2x strength could of done this. (more is better, NOT) maybe o2 sensor wire busted due to snow build-up? any other ideas?
#89 - Anxiety64 - 01/21/2011 - 18:47
shop manager
I am a auto tech with over 25 years experience, there are a few things everyone here needs to know. #1 a universal catalytic converter fits nothing, it has no specific application and in most cases is a waste of your funds. Most cats fail due to misfires or rich conditions. Some fail because they want to. If you are going to make repair yourself at least use a direct fit part that was designed for your application. After reading these post i would be willing to bet there has been more money spent trying to Do It yourseleves than it would have ost to have a experenced tech dianose and repair the problem, or at least get a accurate diagnosis.
#90 - Dwayne - 01/25/2011 - 09:10
2002 Camaro
Over a year ago I had MANY problems with my car. First the fuel pump and then... the cat. I went to replace it, got the part from Advance. When I finally got around to doing it, it DID NOT FIT. I was mad. I took it back demanding an explination. I was told that the car had California Emissions. Thankfully the new one fit. It doesn't matter where you live or where you bought the car, it's one of those things that is a pain in the rear. I just wanted to give everyone a heads up! P.S. The P0420 code just came on in my car again. My heart sank. This one should be fun.
#91 - Briana Jones - 02/03/2011 - 10:28
the boss
PO420 code
solution to the problem getting together and get into the law makers heads in capital to fine the auto makers. set this sensores to a gasoline that most consumers used the gas has change into more etanol and we are paying for this. my price is over 1000.dollars one person they dont lisend but a massive lawsuit they have to look what happend to TOYOTA. we all got prove of this code has bother a lots of consumers sign into theomega2012 dotcom /entry/signin please the coment under 420 code
#92 - Chuck - 02/03/2011 - 13:40
The CHECK ENGINE LIGHT was made to scare women!
I am a used car wholesaler, I sell only to dealers. I have at least 5 cars a year either display 420 and sometimes 430(V6 or V8). I have tried it all:

upstream O2 sensor(bank 1) downstream O2sensor(bank 2)

I have found unless you are getting a code like p171 or p300-p308(miss-fire codes) or possibly a mass air flow sensor code your screwed. In other words a rich or lean fuel code.

However, If the cats are not glowing red. The engine is not over-heating. You are getting normal fuel economy. You have what we call a BITCH CODE. In other words a factory money maker, for scared women.

Funny how it is illegal for a junk yard to sell a cat-converter. No not really, It is all about the cash.

Cat's have in them WHITE GOLD(PLATINUM). So I suggest a little piece of black electical tape to go on the instument cluster, unless you live in a fucked up state like California, well then you are screwed.

Best of luck


#93 - Tony - 02/07/2011 - 01:40
Two months ago I brought my 2001 Sienna to the dealer for engine check light on. They replaced the sensor B2S1 Recheck System. Now the engine light is on again. I went back to the dealer and he said its the Catalyst System Eficiency Below threshold (bank 1)P0420 code. He recomended converter assembly replacement for $1,425. Is this really how much this cost.
#94 - Danny Arm - 02/07/2011 - 21:09
mercedess-c240-m2001(code c1142-01)
#95 - MONTHER - 02/11/2011 - 07:18
Saludos, mi jeep presenta los siguientes mensajes en el escaner


#96 - ARCENIO CUEVAS TERRERO - 02/11/2011 - 12:25
love to fix
98 olds 88 420 cams only in city 10+ lights no highway and no when is park and idling for 20 min.may be catalic con. no codes coils changed rear o2 too found leak air lines never show 420 ,when i fix 2 codes was off ,then shows 420
#97 - bogdan odwazny - 02/14/2011 - 01:18
0420 code - 05 Altima
'05 nissan with 175,ooo miles will not start. The P0420 code is the only code appearing on the reader. Where do I begin to get the car started and the code repaired?
#98 - dave ambrogi - 02/17/2011 - 10:19
#99 - sharron - 02/17/2011 - 16:48
1999 Dodge Caravan SE.159000 po420 popped up. Had oil changed. Service engine soon light came on. Placed my scanner on it and said PO420. Check Engine Light never came on
any ideas tried to get service engine light to go out, no joy. Any ideas
#100 - Edward - 02/17/2011 - 19:50
love to fix
bank#1 refers to one cylinder or all on one side,i check #1 is ok no oil,nice and clean,whats mean #2,bank#1 is cylinder-1-3-5 it is correct,bank#2 is cylinder-2-4-6,i have check#3,#5 for any oil marks thanks
#101 - bogdanodwazny - 02/19/2011 - 01:55
Can a Evap. Emissions System (fuel vapor canister) error eventually cause a P0420.
#102 - Chuck - 02/21/2011 - 09:54
2005 Quest P0420 Code
I just got this code late at night. I have spent the last hour or so reading all the posts. I plan on trying a good fuel with the least amount of ethanol in it, and then a good fuel system cleaner. I am not convinced that I really have a true problem with the Catalytic converter. I will report back and let you know if I have any success.
#103 - Brad - 02/24/2011 - 01:36
#104 - RICK - 03/02/2011 - 19:27
P0420 code 2008 trailblazer ss
I was trying to be cheap and use cheap gas and my service light came on. I read a bunch of post here and one post said to try gas that has less ethanol in the gas. Well I did and it worked I filled up with 93 octane from sunaco and a day later the light was off.
#105 - Earl - 03/03/2011 - 12:02
I have a 98 GMC truck and check engine light came on p0420. I had a tool to check the codes and clear them, I cleared the code and it would come on again, sometimes after a week or more. I called a muffler shop and he said it was the catalytic converter, he has only seen a couple oxygen sensors go bad. I took it there and they checked it, and said it was the converter causing the code and it was not working properly. Yet my truck was running ok. The converter I had on was only 2-3 years old. They replaced the converter and cleared the light and so far after more than a week and about 300 miles the light has not come back on again. They said the converter I had was a cheap brand, I should have know, it only cost about $130. This one was $240 but they said a much better converter than the one I had on before.
#106 - Charlie - 03/06/2011 - 00:53
I saw yours problem with Po420 refers to cataliytic con.I think you rigth but good cataliyic cost $500 white gold in side so the light may come back soon,a year or two I have same problem ,car runs good I have the tool I clear code i think to le go ,problem you put new cataliytic and light may cme back
#107 - bogdan odwazny - 03/06/2011 - 03:10
same problem as above
Is there a spell check here?... It is desperately needed. Otherwise I agree with Tony, the 0420 code is a BITCH. A money making bitch, from the muffler shops to the dealers, to the engine diagnostics to the plethora of 'discount aftermarket catalytic converter' makers/distributers... Even called Toyota and there was a vague TSB that mentioned the 2003-2004 Corolla with 'Reflash ECM and replace converter'..Hmmmm The manufacturer is aware of a problem and keeps it under the hood~ I'm taking Tony's advice ...Tape!....

please pass this on for Toyota owners as TSB's are noted on several other models and years...
#108 - paul - 03/10/2011 - 20:06
My friend if i tell you do spell in french or spanish how far you will go .Do not criticize any one spelling,we here to help each other and share our comments
#109 - bogdan odwazny - 03/13/2011 - 04:57
2005 Quest P0420 Code
Follow up on my earlier post. I did run a tank full of BP Ultimate fuel through, and at almost 1/2 tank used, the code went away. I have since run another tank of Ultimate through and now am just running standard 87 octane fuel from BP, and so far no more codes. I will keep updating if any further issues come up. Hope this helps all of you, check your fuel and run some good fuel system cleaner through your car when you see that code, I recommend XLP Gasoline Fuel System Cleaner. Good luck with your car.
#110 - Brad - 03/17/2011 - 20:46
2002 Intrepid ES 3.5 P0420
I'll try the Sunoco gas first, seems to be the cheapest way out, although temporary. All I want to do is get my MA emissions inspection with no "current" P0420/30 codes, then I don't care for another year. I'll let you know.
#111 - John - 03/20/2011 - 16:20
Good idea first put fuel injection cleaner,empty the tank,then sunoco 94 go on highway for 30 min.stright to the test
#112 - bogdan odwazny - 03/20/2011 - 20:07
code 420
I purchase a 01 dodge neon SE,I'm getting super bad gas mileage drving short distances.I found the trouble code PO420,which means problem with o2 senors, leaking exhaust,cat convert,misfires,etc. how can I be sure that the right problem gets fix?, and how do you reset the computer after fixing it.also pulled the plugs and found some oil deposit on the tip, could this be something else to look at.
#113 - darryl - 03/23/2011 - 01:14
I changed the converter the all sensors and still getting code p0420. What next?
#114 - Jorge - 03/23/2011 - 08:42
#115 - richard - 03/23/2011 - 14:39
I have a 2003 Lexus GX 470, the code P0420 is on. I want to know is because the muffle is broken to course the problem? Please help..
#116 - jeanine - 03/24/2011 - 00:38
gx 470 #1 is cat. converter if replace only good one ,if car runs good live alone.dtc 171 may be dirty maf sensor try to clean very careful ,fuel filter dirty,check air filter is same what 174.po420 how much you pay for cat.conv.Neon burn oil may show 420 how engine runs,air filter if is chocking may be cat. plug light is flashing thats misfiers,to all vaccum may leak look anything crack or loose
#117 - bogdan odwazny - 03/24/2011 - 01:20
i want to see the images code P0420.and the model car camry97 and camry99.
thank you very much
#118 - chanthorn ngeth - 03/26/2011 - 02:39
P0420 in 5.7L HEMI motor
I would like to address some of the questions listed here. The P0420 code does in fact indicate that the Catalytic converter is not functioning correctly, however it also can indicate a bad downstream O2 sensor. Should you just leave it alone? I suppose you could, but be warned that it could cause much more serious problems such as P0300 (random/multiple cylinder misfire) which can cause hesitation, rough idle, EXTREMELY poor gas mileage, and can even cause the motor to fail entirely (tough not permanently... usually). In short, if you get this code, check/replace the downstream O2 sensor and see of it comes back, if it does, there's a very good chance the cat needs replacement. With no symptoms you could just leave it, but know that a bad cat will only get worse, and could clog. A clogged catalytic is a very serious problem, as it will prevent exaust gasses from being able to exit the combustion chamber, and can cause permanent engine damage.

I've had a terrible misfire for some time now, and could not locate the cause. I had changed all 16 spark plugs, 8 wires, 8 coils and even the EGR valve... to no avail. By the time my OBD spit out the P0420 code, the the O2sensor circuit high-voltage code, I had done irreversible damage to my cat. Though the misfire is now gone, I am still suffering poor mileage until I can replace the converter. In short: take the P0420 code seriously and give due dilligence to figuring why it has been logged. It can save you alot of hassle and even more money.
#119 - Colin - 03/28/2011 - 18:14
5.7 hemi have you changed 02 sensor or you going to change,what code you have more then po420,you will notes starts chocking,looseing power,getting clogged.02may take more gus,check air filter.Last six months geting po420 no symptoms good gas mileage engine runs good.i changed back 02 sensor 1.5year ego.comes only in city never on highway for now leave it alone,i had 2 more codes map and egr.mech.said bad sensors i leave it alone .latter i notes crak in vacuum and codes was gone
#120 - bogdanodwazny - 03/29/2011 - 02:14
i have a 98 chevy venture, when i am driving this thing i feel that car shaking or about to shut off but it keep going and some times at stop sign and about to go i feel the car will cut off.
i bought acode scanner which read P0420
1- chang the fule filter nothing
2- plugs and wires nothing
3- Cat.converter nothing
if any body knows anything please let me know
#121 - shanab - 03/31/2011 - 20:04
98 chevy,air filter,throttle body clean ,put fuel injection cleaner try this first if not there is set screw for low idle,you can make higher.may be is overheating
#122 - bogdan odwazny - 04/02/2011 - 02:06
codes P420 and P1136 ON MY 2000 VR6 JETTA
Havent done anything yet?
Where should I start?
#123 - javier hernandez - 04/02/2011 - 02:50
2005 Quest P0420 Code follow up
I have confirmed that the brand of gas does matter in my van. If if use BP, Shell I dont see any error codes. If I have to use a cheeper fuel, Speedway, I get the codes in one tank of gas. I also used a great fuel additive, XLP fuel treatment, and no problems! I can sell you some if you need it. Multi treatment for $15 + sh.
#124 - Brad - 04/04/2011 - 22:45
I have 2002 Hyundai Santa Fe 2.7 AWD, 142, 000 with code PO420, that also gets poor mileage since code began coming on, and frequently goes off by itself(but not as much now. Down to about 12.5 mpg city.
Mechanic said O2 sensors scanned Ok. If Bad cat would not cause poor mileage could it still be sluggish front O2 sensor causing poor mileage and setting the po420 code?
Thanks for any help before I change expensive parts that aren't needed.
#125 - Garry - 04/19/2011 - 22:52
What I usually tell anyone who ask about testing catalyst converters is..........
Look for holes or damage.
If all looks fine, take vehicle for a good run after dark and if cat is getting plugged it will be glowing red enough to easily catch paper on fire. If vehicle is working fine it is more than likely the downstream O2 sensor.
Best thing to do is give vehicle a good tune up and run a tank of high test gas, clear code and see if code stays away.
#126 - Nathan - 04/21/2011 - 04:18
The problem is 02 sensor at the back older cars do not have one ,no po420.What happend if disconnect sensor.
#127 - bogdan odwazny - 04/22/2011 - 01:52
P0420 Code -- Muffler shop said head gasket leaking!!
I had my catalytic converter replaced on my 1999 Chevy Lumina in May 2010. In December 2010, check engine light came on, code P0420... took it to muffler shop where cat was replaced and they checked everything out and told me that my head gasket was leaking coolant into exhaust and that I needed to have the head gaskets replaced ($1600.00). I went to the dealership and had them check my car, after telling me what muffler shop said, and they said that it was a bad catalytic converter, not the head gasket!! I took it back to the muffler shop and had them replace the cat under warranty even though they gave me a hard time, saying the would do it but only this time!!

I recently had my intake manifold gaskets replaced because they had been leaking for sometime, and I would think that dealership would notice if the head gasket was leaking, since it's my understanding that when the head gasket leaks you get coolant into the oil!!! Now, today, P0420 code is on again!!! I'm just wondering if anyone has ever heard of the head gasket leaking coolant into the exhaust system causing the catalytic converter to go bad??? I'm going to the dealership tomorrow to have them check the code and see what happens!!! Any insight into this whole "head gasket leaking" idea the muffler shop had, or are they just trying to get more money out of me?????
#128 - Carl - 04/24/2011 - 18:17
420 code
I have a p 420 L bank 1 on my 04 jeep wrangler,it only has 88,000 miles on it. I really need some help tiring to get this figured out. I am tiring to pass emission.
#129 - Donna Bowlin - 04/25/2011 - 14:50
98 nissan frontier 2.4L
I had a few codes on my nissan. bank 1 sensor 2 which is the downstream sensor. Cylinder 4 misfire. then the p0420 code. Replaced o2 sensor first $55. cleared code and 2 days later back on. Gave same codes again. Checked dist cap,wires,plugs,and rotor. corrosion build-up. replaced all $80. only code i get now is the p0420 which im sure is because of the exhaust leak i have. Ill fix and let all know.
Advice: with high mileage vehicle owners. suggest doing a tune-up and exhaust leaks first. much cheaper to start there plus the tune-up will make your car much more efficient. watch where you buy gas too. run better octane fuel.(gas prices suck i know) Newer vehicles generally require it. especially turbo charged or supercharged engines. 91octane or better. should say on your gastank lid.
I stuck an old rag in the end of my tailpipe while it was idling. heard my exhaust leak clear as day then. (tip from a muffler shop) Do not cover you cel or mil light with tape. although this code usually doesnt cause your engine to run differently it still needs corrected. if something else goes wrong with your vehicle you wont know it because your light is already on. could end up costing alot more in unforeseen damages. Id start with the leak checks and a tuneup on high mileage vehicles. recommended at 100,000 miles anyways.
#130 - Matt - 04/26/2011 - 11:52
97 Honda Civic PO420
After reading this forum, did a little pm on wife's 97 Civic. Re-gapped spark plugs and cleaned air filter. CEL now gone. We'll see how long this lasts since her car has 400K on it!
#131 - Triple_T - 04/29/2011 - 22:23
Saab 9.3 P0420 Code
just had a head gasket oil in coolant..but High pressure in overflow leaking as car cools down...fixed it...heard dripping sound from rear...figured rain in clogged drain...drove car CEL check engine comes on... but car still runs good...Go to my friend SAE Saab mechanic...he reads says Catalytic converter below threshold...but say it may not be converter...he puts car on lift and plugs exhaust and guess what..there was a small tiny button size the could hear it .whistle .causing the O2 sensor to read a different before vs after dont assume it's one thing because it shows a particular code...check the simple stuff looking for holes in the exhaust...then O2 sensor then cat-converter...code readers are just that..they give you a good place to start..and sometimes they are dead on... just do your homework and don't jump to conclusions..
#132 - David Brown - 05/04/2011 - 01:52
I took my car to the Nissan dealer to replace the catalytic converter. I drove the car home and later that day the service engine light came back on. I took it to autozone to have them read the code it still gives P0420. The guy said it's probably teh gas cap. He cleaned off the gas cap and put the cap back on and the light went off. Now the service engine light is back on again.
What is the problem.
#133 - Carol - 05/05/2011 - 11:09
can different grades of gas affect your cat
i have a 2002 grand am gt 3.4 ram air v6 with 159,000 miles... i run premium gas in my car as my choice of gas... i just recently got the cat replaced, and like a week later the same code (po420) came up... i was wondering if different grades of gas can affect your cat or anything for that matter in a bad way??? if anyone could answer it would be appreciated...
#134 - marcus 2002 grand am gt ram air - 05/12/2011 - 09:47
grand am 02 why you replaced you had any problem with the car just po420,I look for answer for a while,replace cat is not good answer,nobody has answer that is a problem???
#135 - bogdan odwazny - 05/13/2011 - 01:14
please car now camry pass all other

1996 camry 4t1bg12k1tu929924 now car pass all other yes


#136 - ali - 05/25/2011 - 09:16
I M getting the sign that says CHECK ENGINE

the code no is p0420. Can you show the parts with the price
#137 - robin mendez - 05/28/2011 - 12:08
1998 chevy blazer
Hello, I have a 1998 chevy blazer that had a plugged cat so my uncle gutted it out. Now my MIL is on for this code. I was wondering if there was anyway to trick the PCM to think that the cat is working efficiently and there are no problems with it. I was thinking about cutting the wiring to the O2 sensor and putting a resistor in between the wires... Could that possibly work?? If so how much more MPG would u expect? Any help would be greatly appriciated.. I am 18 years old and gas is killing me! Im getting about 11 mpg in the city. PLEASE HELP!!!
#138 - dplourde - 05/28/2011 - 20:32
P0420 Code
I have a 2000 Pontiac Trans Am with only 82000 miles, the engine started misfiring so I pulled the codes (P0420, P0704 and P1416). Is there anybody out there that have had simular codes on their Tans Am? Please help!
#139 - Roy - 05/30/2011 - 15:56
Converter advise
A "CAT" Converter should never fail. If so you have some issues. Most of the time it has to due with 1.incorrect timing or fuel mixture 2.damage to exhaust system 3.faulty check valve 4.faulty injector 5.oxygen sensors & the number one cause Faulty spark plug & wires or ignition failures. By the way when you go to the parts house & the read your CEL and they say nothing to worry about, you should be worried CEL doesn't come on for no reason. There is nothing wrong with a high flow cat near a universal cat the reason why the code comes back on is a high flow cat has 1 less brick of filtering material than OEM so use a factory replacement and all is well.
#140 - TED - 06/13/2011 - 09:39
don't throw your money away let someone with experience diagnose your po420 code. you could throw part after part at it and still not get to the source. The first thing I recommend is to try a fuel additive like et100 by e-tech sold at napa that reverses the effect of ethanol in your fuel system.
#141 - ron - 06/14/2011 - 01:53
psycho mechanic
I have an astro van throwing code p0420. I found the flange gasket after the last o2 sensor leaking and 2 bolts broken. will this cause the code ????
#142 - JOE - 06/21/2011 - 16:18
psycho mechanic
follow up comment to question #142....I cleared the code and had the van E checked here in OHIO and because there were NO codes stored in the computer they had to perform a tailpipe test and the van passed with no problems so I don't believe the cat is faulty and the problem is with the leak at the flange. thanks
#143 - JOE - 06/21/2011 - 16:26
2001 honda civic i work head gasket warter pum,radiater,thermostar, run 20' then over heat temp 260F 2fan run good then code po420 is on .that coud be becau?
#144 - david - 06/24/2011 - 02:13
#145 - SAM - 06/25/2011 - 14:47
#145 You mean both wires replace
#146 - bogdan odwazny - 06/26/2011 - 01:38
repair information
i need repair procedure for c1237,c0200 on toyota corolla 2008 i will appreciate
#147 - lordleo - 06/27/2011 - 14:17
Honda civic 1999
I just bought the honda civic Lx 1999 and it have 158000 mile on it, now the engine light on can you tell me what the cause. at the auto shop show me the code of that light on is P0420. Can you give me some adviser to solve this problem. Thank you in advange.
#148 - david hang - 06/28/2011 - 10:25
I have a Jeep Classic 1996 took to advance auto n it read these two codes what should i do? cod p1297 n p0420
#149 - celines saltares - 06/30/2011 - 17:31
2003 grand Am
I replaced the Cataltic converter about 2 years ago. Could it be going bad again
#150 - Ricky - 07/08/2011 - 19:20
I have a 04 Passat wagon. 1.8T. with P0420 code. What do i need to do?
#151 - Lucian - 07/17/2011 - 21:51
My Chevy S-10 has been giving me fits on this code since last Nov. Do you think in all this time that ANYONE would think to utter the words 'California Emissions'? Well thank God some finally did! Now I can order the one piece pipe and bolt it in myself. No more messin around with the muffler shops or friends who think they are back yard mechanics.


Get it and get back on the road!
#152 - A Carpenter - 07/18/2011 - 23:43
no performance
I have a 2007 subaru STI, this morning it all of the sudden stopped boosting however threw no check engine light on the dash. I automatically assumed it was the turbo however, i ran the special emissions tool you can get at autozone and it is telling me this same exact code...could my catalytic converter need to be replaced?
#153 - Taylor - 07/20/2011 - 16:14
I have 99 chevrolet malibu and it's given me the code po420 and po452 i just change my catalytic converter and when i did it that's the code it gave me it runs find with the codes but i would like to know what else can i change that would help me make these codes go away.....
#154 - Shawn - 07/25/2011 - 17:41
P0420 on 2005 4cl Camry
Took it to dealer, they chart 100$, said code was P420.

Then took it to MIDAS i usually go to, and they said the best thing is to replace o2 sensor on top of the manifold as the voltage was not normal. I reluctantly agreed, but light came up again.

Stuck now. Please help. I want to determine the root cause myself. What all can I do?
#155 - KD - 07/28/2011 - 11:50
po420 on 1999 kia sephia
just installed new header and exhaust system and now the engin light is on. is there a way to shut it off? ie dummy switch? if i put a bit of resistance on teh sensor above the cat will it shut off? not trying to run stock ...want the Quick fix for racing parts plz.
#156 - Allen - 07/29/2011 - 00:29
tacoma owner
I had same problem. I replaced 02 censor between engine and converter and still check engine light on. I checked exaust mani and found a crack. I replaced it, also flushed radiatior anti freeze it was rusted and dirty. Now light is gone. It works for me.

Good luck!
#157 - john - 07/30/2011 - 19:57
Acura Integra 1.8 1998
what does this code mean: po420 catalyst efficiency low bank#1
#158 - IMG - 08/01/2011 - 23:29
Helpful solutions
I hope this helps, I have a 2000 cr-v lx. My bi-annual smog exam came up in the dreaded California. From what I assessed both online & from talking to smog mechanics i replaced the catalytic converter, & 2 oxygen sensors I replaced the oxygen sensor before the cat. Just in case I'm pretty sure only the downstream oxy. Sensor was bad according to the odb Codereader.Also topped off coolant & changed the oil. I cleared the codes than put about 50 freeway miles on the odometer I believe The comp needs to perform a drive cycle. Took it to simplesmog it passed, mil hasn't come back. Hope this helps. check engine replace, sparkplugs,airfilter.etc. This all helps. Goodluck.
#159 - Filtertech - 08/03/2011 - 19:36
Need Help!
I have 04'BMW325ci and for some reason i come up with this codes P0420,P2237,P0171,P0174,P02240 does anybody knows what's that means please help me solve this problem. Thanks!
#160 - Viktor - 08/04/2011 - 13:52
Great article.. Lots of questions but no ANSWERS..!
It will be much better if some CATS experts answer those questions in order. Maybe, they got their answers by email? If so, is there a link to go and check them out. It would be a great.. GREAT... informative source. But, it really nice article thou...
#161 - Mike - 08/04/2011 - 15:48
My 2001 jetta has a strong gasoline smell
I have a 2001 VW Jetta 1.8T Wolfsburg Edition and for about 6 months it has had the service engine light on and a strong gasoline smell. I had it checked at an auto-zone with a scanner and it came out to be a misfire code for all 1,2,3and 4 cylinders according to the guy who told me about these codes he said it was all 4 of my fuel injectors so i got them replace and after i got them replace my service engine light still on and it still has a strong gasoline smell inside the car and trunk. It has also consuming me more gasoline and every time I'm driving my car it skips when i push on the gas and the service engine light blinks. I have put a lot money on this does anyone out there might have a solution that can fix my problem or any of this type of symptoms on their car and what was it?
#162 - Jesus - 08/09/2011 - 12:26
I have 2003 Toyota Sienna it is P420 problem, what is is, how could be it fix and who will be the best to fix it.
#163 - Zac - 08/09/2011 - 17:33
just replace the fuel pump and chaeck engine light is on and gas hand does not work.
#164 - nicole - 08/11/2011 - 19:04
Scotty knows the best!
Hey friends!
I found answer here. Scotty, the Mechanic describes how to resolve this issue in his YouTube video.
#165 - Alireza Yami - 08/16/2011 - 22:47
Possible bad catalytic converter
Only when I go at highway speeds for more than an hour, at one time, does my check engine light come on. This happened 2x's and each time the light goes out by itself (after a week or two) of local driving. Toyota told me that the cat converter code came up and advised me to change it. The check engine light is not on now. Does this mean that the part is bad? Could it be the o2 sensors or something else?
#166 - Rhonda - 08/18/2011 - 15:24
I have a 99 gmc sonoma auto with the 4.3l vortec engine. Check engine light comes on for a few days then goes away and comes back on. i bougth a scan tool and the codes p0113 (IAT high output) and p0420 (we all know its for the cat) checked the iat sensor. cleared the codes, now only the infamous 420 stays on.

I read a few and I strongly believe that the gas has something to do with this. Reason being, my brothers 03 camry's light is on and a mechanic scan is and it has cat problems. I also scanned my girlfriends car and its threw a code different from p0420 but the code is for cat performing insufficienly. So I wonder why? only thing these 3 vehicles have in common is gas. Ima check for leaks and Ima swith to shell gas 93 and see what happens.
#167 - Luis - 08/18/2011 - 23:05
03 altima
I have an 03 altima 2.5 4 cyl. with 170,000 miles that was basically given to me because my brother was tired of bringing it to the shop. One of the cars previous problems that I was told about is it was miss firing. The car also stalled on him a couple of times. Before I received it, the O2 sensors, 2 coils, spark plugs, and battery was replaced. There was a couple of other minor things changed. The mechanic claimed that the car stalled because of a short on the headlamp. When I received the car, the check engine light was on and read, you've guessed it, P0420. As long as the car was ran normally, it ran great. However, when I drove the car hard, it would rev up to 5000 rpms and would not shift unless I let off of the gas a little. Because of the know problems with nissans cat. converters/exhaust manifold on the 03s, I went ahead and changed it (which was damaged on inside) and reset the computer. I've been driving for 3 day since and the light hasn't turned back on yet. Like before, the car still runs good under normal driving, but if you get on it a little hard, it has the same problem as before. The only difference that I noticed after changing the cat. converter, is that the car has LOST hp. After the converter was changed, I drove it really hard to see if the problem was fixed. Twice I tried pinning it from a dead stop and the car nearly stalled on me. I'm trying to decide what I should check next or if it's even worth my time and money to try to fix. I was thinking about changing the fuel filter and checking for clogs in the exhaust. Any suggestions on what to do next?
#168 - Ryan - 08/19/2011 - 05:31
For #164 - Nicole
It sounds like the sending unit isn't working. It could be part of the pump or a separate piece that failed.
#169 - Elaine - 08/19/2011 - 14:39
#170 - AL GREEN - 08/20/2011 - 16:22
2004 Impala P0420 code
I had this code in 2009 and did not do anything about it. I still got 30mpg on the road and the car ran fine. I had to get the car emissions tested in 2010 so I had the catalytic converter replaced by my mechanic. The light went off for 10 months and then came back on again with the same P0420 code. The mechanic replaced the converter under warranty and all was fine for 500 miles and the light is back on. The car has 195K on it, but it runs just fine. Anybody experienced the same problem with your Impala?
#171 - Mark P - 08/22/2011 - 20:21
I've replaced both o2 sensors and had a recent oil change but my check engine like is still illuminating. DO you think I may need to replace my catalytic converter?
#172 - Honda Civic EX 99 - 08/23/2011 - 14:14
chevy malibue
what would be the cause of a multiple misfire in a 98 chevy malibue after changing the spark plugs and the fuel filter.
#173 - grace - 08/27/2011 - 14:13
chevy malibue
Moduler and coils is not cheap.Goodlack
#174 - bogdan odwazny - 08/30/2011 - 01:49
CATACLEAN by Prestolite
CATACLEAN By Prestolite is the only additive made to fix the P0420 code. It Cleans Catalytic converters and O2 Sensors. I have a friend who had a P0420 on a '99 Dodge 1500 and it also wasn't running right. He poured that in and it fixed it after around 30-60 miles. Different Magazines have also reviewed it.
#175 - Derwin - 08/30/2011 - 10:59
I see all the questions , but none of the answers can be read. ? it keeps bringing me back on the beginning of the page. Why
#176 - franck - 08/30/2011 - 12:19
cat question
i had the code for it im just curious does a pontiac vibe have 2 cat converter parts? or did i get screwed?
#177 - kozy - 09/03/2011 - 04:04
Replaced the catalytic converter when the exhaust pipe downstream of it corroded irreparably - big mistake. Purch'd a direct fit piece online for ~$250 and installed myself (after lots of choice words while getting the rusted/fused bolts of the old part off) instead of paying Midas about $1200. Should have brought the job to a freelance mechanic who knows how to cut out the corroded parts and leave the old cat converter in place, which no doubt had way more Pt than the replacement. Now, a year plus later, the p0420 code comes on; and when erased, it comes on after about a week or two of local driving. Still sorting out whether the O2 sensor and/or the cat converter is/are duff. Any other ideas to try?
#178 - DJS - 09/04/2011 - 11:37
California Emissions
If you replace you cat converter with a universal it will not resolve your Issue. you have to Install an cat converter that has a calif emission lable. FYI this will not be a 200 converter either. Price tag of 400 and up for the right converter. In order to get the right converter look under your hood and find the Emission number for your make of car in calif. Only in calif we have tight Emission standereds??? call Pet boys or kreagen on ur cell phone and they will give you your code number or go to the dealer. Rember your cat converter must be a calif emissions approved cat or your light and smog will not pass. Your check light will remain on too. Your christian cheif mech3. If you go to the dealer after you put a universal Cat on your car they will not tell you that you got the wrong cat converter? They make money and have you return back once you replace it with another wrong universial Cat. I have seen the same guy put 4 cats on all not meeeting cal emission. He finaly asked what is the cost of the dealers cat $580. We put it on and issue resolved. Hope this helps with your cat issues
#179 - Pete - 09/06/2011 - 22:40
so i did the obII check p0130,p0133,p0125 popped up. i changed my radiator since i had a crack,thermostat,both o2 sensors were also changed started up engine idoling rough p0420 popped up? my truck is a 1999 toyota tacoma..
#180 - manny deguzman - 09/10/2011 - 01:25
01 Eclipse P0420
I had two codes thrown for cylinder misfires, smelled sulfur (rotten eggs) occassionally while driving, then car developed a rough idle at stops (which suddenly started happening and then stopped about a week later), and it wouldn't shift correctly when taking off from stops occassionally. I took it into the shop and sure enough the catalytic converter needed to be replaced along with the O2 sensor. Also, at first my check engine light would stay off for awhile after clearing the code but always came back on until it stayed on consistently.
#181 - BH - 09/13/2011 - 23:51
cat tech
remember: fix the problem that caused your o/e unit to fail ( a converter is designed to last the life of your vehicle as long as the engine is in factory spec;'s and there are no moving parts inside to malfuntion )or it will destroy your new one too. and always take your vehicle to a A.S.E. CERTIFIED emissions specialist or YOU WILL BE SPENDING MORE MONEY THAN YOU HAD TO AND STILL HAVE THE SAME PROBLEM TO DEAL WITH!
#182 - cat man - 09/14/2011 - 10:16
please 1996 camry clean titte car 4T1BG12K1TU929924 clean now yes order
#183 - ali - 09/17/2011 - 12:55
i have 2001 tahoe/surburban,the engine service light just came on,my truck runs so good,im not sure what the reason would be,there are no blinks when the light came on,it just turned on.anfd is staying on.any ideas
#184 - amber - 09/17/2011 - 20:25
honda civic 1998 stalling with code p0429
I have a 1998 honda civic dx with the code p0420 bank 1 my car stall without warning i was wondering if a bad catalytic converter could cause it to stall like that i already took it to get looked at he had my car for almost 2 weeks and could not get it to stall i picked the car up on my way home the check engin light cam on and that is the code I got the car did not stall for a week and is now stalling again help please
#185 - becky - 09/19/2011 - 15:56
2006 Nissan Quest Code PO420 the van has a roar to it when we drive it down the road that seems to be coming from under the van right about the front passengers still has good power just under 89000 miles why is it so hard to find out what the trouble is with this code? instead of 4 or 5 things that it could be why can't it not be known what it really is?
#187 - L M B - 09/24/2011 - 15:03
before replacing your cat for p0420
it has been my experience that when replacing a cat for p0420 you must use factory cat. the paperwork with your aftermarket cat will tell you it is certified but it appears that the cat manufacturers certify the cats themselves. this is not a federal epa certication. i am a tech of 28 years and have tried many aftermarket brands of cats with little success. i do not have problems when using oem cats. i went as far as calling an aftermarket cat manufacturer and asking about federal certification and they danced around question. (i was taught what cat certs are in class but forget if its federal cert or c.a.r.b. or something else, could probably look it up but just trying to help some folks out here, you get what you pay for, a cheap cat is cheap for a reason) always checks with your dealer over the phone for latest pcm flash updates, some cars require a pcm flash that widens the parameters that cause this code, you will need your vin# when you call
#188 - bfaestel - 09/29/2011 - 12:32
i have a 2007 toyota corolla le and my check engine came on and it say po420 wat do i have to look for.
#189 - Ish - 09/30/2011 - 22:14
I have a po420 code and I replaced the rear o2 sensor and had the code removed. With in 50 miles the check engine light came back on. I had it re scaned and again po420!!!!

The car seems to have slight miss when running. I do not want to to trial and error as this could be cost prohibited and as we all know times are not that prosperous. I have a 1996 ford exployer sport v6 4.0 4 wheel drive with a 5 speed. Also what is the differance between a scan and diagnostics. Can any one please help me! bob
#190 - robert - 10/01/2011 - 15:48
#188 read this and try to figure out,car is 15 years old,if do not have any problem live alone and drive until die.
#191 - bogdanodwazny - 10/01/2011 - 23:15
Mexican Accord
I'm from Mexico and I was told that no Accord built in Honda Mexico has an EEGR? Is this correct? I recently bought mine. It is a 07 Accord SE (2.4L)
Besides I got a P0420 code, but it is coming for a while and then it goes by itself. What does it mean?
#192 - Horacio - 10/08/2011 - 14:20
after change of catakytic converter what to do next
I changed catalytic converter on 2000 mazda millenia wont run any better now what
#193 - kim - 10/15/2011 - 19:22
Reply to post #128
Depending on the type of coolant your car uses, leaking coolant into the exhaust will kill a converter and the O2 sensors. The Glycol based coolant (the green stuff) has a high silicon base and will coat the O2 sensors and wont burn off, and make them read incorrectly as well as coat the inside of the converter. I don't recall if the red coolant has the same problem or not but ANY coolant leak should be addressed as it WILL lead to expensive repairs if ignored.

As for high ethanol concentrations in fuel causing PO420 I can only surmise that it's possible due to the different oxygen content from the burned alcohol in the exhaust than non-ethanol fuels but I suspect a trained tech can spot the issue if he looks at the data with a scan tool- the fuel trim (block learn/ integrator) values will be out of norm (but marginally within acceptable specs), as they will set from the front O2 sensor readings that will ALSO be reading the results of burned alcohol in the exhaust. As for fuel trim being set from the rear O2 sensor- I dunno, but if it is it should throw the front sensor into spasms that should set an O2 sensor code.

Everyone should keep in mind- an OBD2 code is a generic trouble code output by a special computer designed to interpret the powertrain modules codes so they can be read by emission testing facilities, it is NOT a code to indicate what part to buy, or to diagnose what to fix. It is a code to indicate to a trained tech WHERE to look for a problem. The real diagnostic codes are in the PCM that cannot be read by a generic code scanner.

Yup, I was a drivability/emission tech for 20+ years, so I have a pretty good idea what I'm talking about.
#194 - MaddogK - 10/16/2011 - 18:52
95 Ford Taurus
My car used a half tank of gas in 52 miles...there is no visible leak. What else could be causing this?
#195 - Connie - 10/17/2011 - 06:09
Fix for error code P0420
#196 - Terry - 10/22/2011 - 16:42
I am the person #190 with one of the latest post with a po420 code for a 1996 ford exployer sport. After horror stories from many mechanics I ran across a site.... discount converter dotcom,,,, They helped me with great service and prompt reliabe shipping.
I purchased a direct fit converter and had it installed and my car no longer has a po420 code and passed the P.A. emmessions inspection. I recomend that any one contact them via phone and they will help you. Robert
#197 - robert - 10/22/2011 - 18:17
what side of my 2005 toyota highlander limited is bank 2 of the cat converter on?
#198 - Tony Fronz - 10/28/2011 - 10:50
Error in p0420 and p0430
I have a lexus gx 470. And the lights are all on! The mechanic says its the p0420 catalyst system below threshold bank 1 and p0430 catalyst system below threshold bank 2.
I have to get new catalytic converters, but just cant find any! Do you know where i can buy it online?
#199 - Marilyn - 11/07/2011 - 22:15
the P0420 code refers to the converter not working up to standards. It's those "standards" that are causing the problem.
My 2002 Subaru Outback wagon had the converter rust off at the flange. We replaced the converter (by Monroe Muffler)and two or three days later the CEL went on. Took it back and they reset the CEL. Worked find for a few months then came back on just before we needed the car inspected (new york).
My mechanic informed me that the car has two sensors (Monroe stated both were O2 sensors). Wrong! The front sensor is the AFR or air fuel ratio sensor and the rear one is the CMS or converter monitor sensor.
Apparently the replacement converter installed by Monroe Muffler is not up to the California "standards" that is programmed into the computer on the vehicle. This means that the converter needs to meet California "standards" here in New York. No way around it.
#200 - Richard Koral - 11/08/2011 - 14:46
toyota corolla 2009 M
P0420 catalyst system efficiency below threshold bank. what do i do to this issue?
#201 - uche vincent - 11/11/2011 - 09:59
enhine light
hi i have my engine light on after i changed the two o2 sensors what else can be my problem. over the summer i had my fuel injecter harness completely burnt all because the wire was laying on the intake manifold and dertroyed the wires and the computer and ever since that happen everything has been down hill. can you please tell me what it could be i am getting codes 0420 and 1684.
#202 - John OBrien - 11/11/2011 - 11:43
I have a 2003 Chevy S10 with 142000 miles. My check engine light is on again. It was on before and had a p0300 so I changed the wires, plugs, cap and rotor. Also used a better grade of gas and put in an additive. I erased the code but it came back on within a few days. At that time the truck was still running however now the truck will not start it is trying but wont start. I dont know what I should do now. I hate to take it to a professional and potentially waste a lot of money. Thanks for any help you can give.
#203 - vee - 11/12/2011 - 18:52
catalyst system failure
code p0240....B1 catalyst failure. I was just wondering which one is B1. I have a 2006 Nissan Maxima and they said i had 2 converts and B1 failed, so my question to you is B1 in front by the transmission or by the mufflers?
#204 - mary ann money - 11/17/2011 - 19:51
P0420 and CEL
#207 - Bob - 11/21/2011 - 02:30
Catalytic converters can be expensive, and if you need to get through the emissions test with little cost, a "fix" is out there for P0420 code. Subaru dealers have a fitting that goes between the O2 sensor and the pipe. It moves the sensor out of the exhaust stream. This does not fix the problem, and is not legal for street use, but it does clear the check engine light. Most sensors have the same type sensor, so even if you don't have a Subaru, it should work, and it costs under 20 bucks. I have seen it work on S10 Chevys.
#208 - Freddie - 11/22/2011 - 14:37
CAT Replacement Often a RIP-OFF !
My 2.2 L Sunfire recently popped a 420 code but no others, so I'll perhaps foolishly assume that the 02 sensors must be working or some other (ie bad sensor) codes would have shown up too.

Since too many bad, greedy, or simply incompetent mechanics & garages also know
that they'll make even more money changing out your perfectly good parts (especially those expensive sensors and CAT's !!), they often seem to miss out on that point entirely.

In any case, it seems that since P0420's can often result from incomplete combustion
issues, whether from bad (or ethanol spiked!) gas, spark misfires, leaks & the
like, you'd think that the easiest and cheapest way to go would be to:

#1)inspect for any intake/exhaust leaks
#2)inspect the sensors & connections
#3)try running a tank of good gas with higher octane and no ethanol additives
#4)clean up your battery terminals (and as a bonus, removing the power momentarily
may also clear the code)

Next, try a tuneup (sparks, wires, filters, & check coils) to see what happens. This way you won't be paying through the nose to change out an expensive CAT or sensors for nothing, and even if you eventually have to, you're only doing some simple and
inexpensive stuff that you'd normally be doing anyways.

I also agree that an OEM CAT should last the life of the car, and would not likely
need to be replaced unless contaminated with oil or coolant (glycol,silicates) as a result of some other more serious engine failure. You might also try using some CAT cleaner (CATACLEAN By Prestolite) before deciding to replace it.

LOL to the "Bitch Code" diagnosis. The 0420 code sure puts a lot of money into dealers and garage owners pockets for useless and expensive CAT and parts changeouts. So beware.

As an aside, I've also heard that using too much gas line antifreeze (methanol) can
sometimes provoke the code. Thanks to all who posted their P0420 fault code experiences and fixes. Now I'm off to go try out my own advice, and I'll be back to let you know how it works out.

Hope that this post has helped someone. Good luck to all!
#209 - Roberto - 11/25/2011 - 16:24
Do it yourself costly guesses
As a Veteran Mechanic and Tech that owns and operates a shop it seems like a great thing when people give each other advice online with no tools or training. Most likely a PO 420 should be properly diagnosed before making repairs. The cost of tools and training is expensive and people that guess do not have to pay for these types of processional items. When reality sets in people whit quality diagnostics are worth more than parts changers will ever be worth. Look at a shops track record and support the good ones. This type code is very difficult to repair. Getting all of the routine systems in order is great advice. The poor tune, misfire, vacuum leak, bad plug, wires, intake leak, coil failure, exhaust leak, all have something in common they ruin catalytic convertors over time. This is monitored by the rear oxygen sensors and is feedback to the processor. If the rear O2 or oxygen sensor(s) is switching like the front it will set the code. Oscilloscope and scanner and thermometer and a brain that functions can diagnose this problem. It is the rule with this type of a code that more than one issue gets the car to this type of failure. It is the exception when only one thing causes it.

1 - fix engine mechanical systems
2 - fix routine systems issues IE. charging system, tune, exhaust, vacuum, EGR, coolant overheat, fuel pump, catalyst etc
3 - fix computer control issues o2 sensors are preventive after 8 years or 80 thousand miles - why ruin cats?

Always check for TSB's (technical service bulletins) dealer or manufacturer recalls and computer flash updates as part of the repair process.

Do you think it is possible that most mechanics are great at what they do and not great communicators? Is it possible that if you had to spend 40-60 k on tools to get a 15 to 20 dollar an hour job that a little respect instead of distrust might go a long way? Give a tech a break. Mechanics fix cars to make a living and help people. PO 420 is one of thousands of possible codes. it is not specific on purpose. Let professionals help you save money, pay them for their time and say Thanks!
#210 - Ray Payton - 12/01/2011 - 21:12
i have changed my cataylic converble twice and it keeps burning out,i have changed the rear 02 sensor as well,any suggestions on what the problem might be.
#211 - lee - 12/04/2011 - 21:23
p0420 code
Please, i need some help with this code, p0420. It keeps appearing even though I've got changed both the O2 sensors. My car is a 2003 Nissan Altima 4 cylinders. it has 130k miles. I'll appreciate any helping comment. Thanks
#212 - willie - 12/04/2011 - 22:51
Ran Gas Tank the P0240!
Ran my gas tank dry a couple times recently. I ran completely dry. Filled the tank with one gallon. Drove home. Next time I start my car, I get the Check Engine light, Traction Off light, and the VSC light. Three lights just for running the tank dry.

So I have a 2005 Scion xB.
I'm guessing either the cat is plugged or something is plugged up with gunk. This would then read in the system as "high fuel pressure" under the generic trouble code P0240???

How do I fix gunk in the upstream fuel system?
#213 - Gabe - 12/05/2011 - 22:59
three not readies
i have 2004 nissian titan O2, catalyst, and evap not ready no i did not disconnect the battery and i have driven the drive cycle many times no codes any ideas oh the iat is part of the MAF
#214 - warren roddy - 12/06/2011 - 09:22
My truck has this code do I have to replace the catalyst converter?
#215 - edgar - 12/09/2011 - 19:45
replace cat now p0300
i replace the cat on a 2001 honda crv, now i get a multiple misfire code but no misfire, happened 10 min after starting with new cat and wont go away anyone had this problem
#216 - chris - 12/10/2011 - 09:58
have the p0420 code, have a rattle when car starts for a few minutes till warmed up some, sounds like rocks in the i right assuming the converter is bad and that is the rattle?
#217 - rick negrea - 12/11/2011 - 17:20
2001 toyota camry - 2 yrs ago got the engine light, took to toyota and they said it was the cat conv. Light went out after 2 wks. This summer it came on an d stayed on. Replaced with universal cat. Both sensors were also replaced. After 3 wks. light came on. Went to Auto Zone and they turned light off. Lght came on again with Code P0420. Help Please
Been going back and forth in the last 2-3 wks. Cat & both sensors are brand new.
#218 - dorothy - 12/12/2011 - 17:52
I have #190 and #197 that I recently posted. Please teke my advice and at least look at the web site I recomended. Call them and they might help as in october I was having panic eppisodes over this as my car was almost side lined due to pa inspection. I called and after talking to them they sold me a cat. It is december and my car has legal inspection stickers as it has passed inspection. The people that are stateing an after market converters do not work are wrong!!!!!!!!!!! I have no investment in this company but they helped me after a great deal of money and stress from this PO420 HELL CODE.
#220 - robert - 12/17/2011 - 19:21
I have no idea what a p0420 catalyst efficiency code was. It is great that you can go online and read more about this. I think it will take us to the dealer and see what they have to say about it. But, it is nice to have information on hand to make sure you are not getting ripped off.
#222 - wheel loaders - 12/21/2011 - 15:52
2006 Honda Odyssey p0420
My malfunction indicator light came on about 900 miles ago. I took it to Autozone and they told me I have the catalyst efficiency below threshold (p0420) trouble code and it is probably an O2 sensor. On the advice of a mechanic friend I put a can of Seafoam in it with a tank of super unleaded and a can of STP octane boost. About 100 miles ago the light went off. I'm hoping it doesn't come back. Wanted to pass this along in case it might help someone.
#223 - JJ - 12/21/2011 - 21:29
2003 toyota corolla
Read most of the comments and want to ask a question, do O2 sensors have a life span? My understanding is that O2's have a live span of about 1000 hrs,depending on how you drive that can be about 100,000 miles or so is this correct?
#224 - bill - 12/28/2011 - 00:28
01 crv p0420
Any advice would be appreciate!!! Code read p0420 on crv.
Took car have dealer check, it's the cat convter. Cost $900+ labor. Damm!!! Don't have that kind of money laying around. But my problem is the car gives out poor mpg 19-20, sometime 21 if I'm lucky. This is base on 80%-90% hwy driving and more. Check and read, car suppose to be 25 hwy and 20 city. I seen other get 30 mpg+ on their crv on hwy. how or why is my giving out low mpg. Did some research and found that it could also be a o2 sensor because bad one can decrease mpg by 20% -40%. I mean WoW that's huge. But not sure if this is the answer cause I don't want to spend a ton of money and turn out it's still gives me low mpg. Anyone ever have some issue like this or similar. FYI, did tune up already. Plugs, wire cap and rotor, change oil, and flush tranny. Thanks for any help.
#225 - BoB - 12/31/2011 - 01:01
2003 Toyota sienna
I have a 2003 Toyota sienna. It's showing the code P0420.
Recently i got its transmission changed. And got new tyrods and bolt assembly. And within 20 days it's showing the check engine sign. It's for CAT efficiency below threshhold.
I don't understand what's the problem. Every month it shows some problem. The miles on the car is 119000 miles so far.
Could anyone tell me where is the cataLic converter in the car and can we fix ourself. I have spent more than 4000 dollars last year for is car and I don't want to apennines any money .
#226 - Bob - 01/03/2012 - 16:30
Need help with 1999 honda civic LX miss fire #1 and #2 cyclinder
#227 - Darrell Spence - 01/05/2012 - 11:38
Honda Odyssey update from 3 posts up
I posted a few posts ago about my Odyssey. Well I have about another 1500 miles on it and no light still. The Seafoam, premium gas and octane boost seems to have done the trick, although I think the Seafoam is what really fixed it. Try it if you haven't already. Good luck
#228 - JJ - 01/05/2012 - 20:47
2003 bmw m3
hi i hav a 2003 bmw m3 and this same code came up. i hav have aftermarket dinian headers so i hav no cat converter. wat is the best thing to do? should i just replace the sensor. plus my car wont go into gear ex. i put it in reverse it will go into reverse but when i press the pedal it just stays in one spot and revs same with all the other gears. please let me know what is the best soulution for this. thanks
#229 - romi - 01/07/2012 - 13:22
2003 toyota corolla
Had a p0420 on my corolla, and have a scan tool with 3 color emissions status and it was yellow. Check exhaust system ,ok , hooked up scan tool and upstream o2 changed alot, down stream was steady, ok . Changed upstream o2 with new bocsh unit 99.00 $, reset pcm to clear code, drove for 30 miles or so, took to test station and it passed.Plug in scan tool , emission status now green, problems solved.
#230 - Bill - 01/07/2012 - 23:31
Checked my 2004 Buick Rainier with a DTC Code Reader and got back PO106 and PO128, what do these mean?
#231 - Ramona Jarrell - 01/09/2012 - 18:40
2004 Chevy Silverado Stepside 5.3
OK I year ago, I never got the code but heard noise turned out to be my cats bouncing parts around in the cat. Replaced with after market cats which are warranted 5 years. I also change the spark plug not the wires. A couple of weeks ago, I got the PO420 code. I changed the OXYGEN down below the cat on driver side, reset the code, it came back on. I put in Chevron premium gas nothing help. Check for leaks, nothing, check for vacuum lines, nothing. I change the spark plug wires and noticed a big difference in power. SO I was about to replace the cats until I read the post above.

I put in one gallon of Lacquer thinner from home depot $19.00 and one bottle of Chevron Pro Guard from Costco $16.99 per case (6 pack) I reset the code went to Sacramento via I5 at 70-80 miles and hour (One HOUR DRIVE) and then came back after doing my business up there. It now has been two weeks, no light / P0420 code, and normally I would get the light on right after getting on any freeway or driving hard. The Lacquer Thinner was the trick. I have 1600000 miles on the engine. I was very skeptical, and pray about the Lacquer Thinner, because this sounded so stupid, and I thought I could ruin my engine even my friends were waiting for my engine to blow up. I have a very strong running truck, I will give it gas and end up spinning my tires by accident. This really works.

I would check the obvious,
Leaks, vacuum hoses, battery connections, spark plugs, plug wires, but the good one Bosh. Then after all that put in one gallon of Lacquer Thinner, and one bottle of Chevron Pro Guard, cleans the fuel injectors.

The OXYGEN sensor cost me $70.00 at Grand Auto, it did not help. Try the cheap stuff first, then go into the more expensive stuff.
#232 - Jerry Syrovatka - 01/09/2012 - 23:16
2003 honda civic ex 1.7
I have been telling with this code P0420 for about two months now and I can get the engine light to go way just by adding sea foam injector cleaner and running premium gas, so that tells me that I need to pay more attention to the 02 sensor on downside or behind the catalytic converter or spark plugs.So now after checking for vacume leaks and cleaning connectors. And also check your cold air intake hose for leaks use zip ties to seal up intake hose if needed. You have to get the little stuff out of the way. I will update after I bench test the 02 sensor and or replace spark plugs. The End for now ?
#234 - mikefg - 01/17/2012 - 09:36
oxygen sensor
I have a 2004 GMC Sierra V-6 and i wanted to know why auto part sites list a front and a rear sensor. The front sensor is always a few dollars highter. Then some sites have them with the same part number front and the rear at the same price. Is there any difference in the to sensors?
#235 - William Beale - 01/25/2012 - 23:18
how to fix p0420
here is a good idea for trouble shooting the p0420 code if your vehicle is otherwise operating normally:

1. Check under the hood and car for any air leaks, such as intake manifold, vacuum hoses, exhaust manifold, exhaust leaks

2. fill up with premium gas from chevron, shell or BP (i like the techron@chevron)

3. drive the engine hard and under load - like up a hill for an extended duration- sometimes the cat can get gunked up with stuff it it hasn't been driven to full warm up consistently

4. if you have an after market cat back exhaust look into making an O2 sensor spacer out of some washers. You can find directions by gooleing. the theory goes the higher flow rate causes the efficiency test to fail. Moving the sensor back a bit will solve the problem.

5. This code can also come on from burning oil if you have leaky valves or a worn motor. If that is the case, replacing the cat will only end up with another failure down the road as the burning oil is what is ruining the cat.

#236 - nathan schiavo - 01/31/2012 - 12:08
2000 Toyota 4cyl p0420 code intermittent
My wife's car check engine light comes up sometimes. It will come up for awhile, and magically disappear with no apparent reason. I bought a new cat and right before I was about to put it in, the light went off. I used to think it had to do with when I filled up, but I don't know. Any suggestions would be great . I have a scan tool, and I can see live stats, but I don't know what the two o2 sensors should be at.
#237 - Joel - 02/05/2012 - 20:51
i have a 1999 Nissan Altima an i need to change the timing chain an i never did it before i need an idea
#238 - assan - 02/16/2012 - 15:13
Hi if anyone can help he with this code , I have a 97 328 is coupe ,I replace my 02 sensor with two used ones, an a few days I got the p0420 , can this cost by have bank 1 sensor plug up at bank 2 I ask this because I never do this work before , thanks to who all can help
#239 - Renny - 02/19/2012 - 12:02
fixed my 420--read this.
I read that vacuum leaks will cause a 420 code, so I looked around... the hard black vacuum line that comes from the main tree goes over the brake booster on the driver's side and connects to a smaller tree. On my car, it was disconnected. I had to take the smaller tree off (after removing the windshield wiper and pan) to get my hands on it to squeeze the line back on.

I reconnected everything and took it for a test drive... the check engine light went off after about half a mile.

Buuutttt... the car quit on me at the next red light, and then would not start back up at the autozone (where I returned the crankshaft sensor they sold me earlier today to theoretically fix the 420 code).

Looking into a new fuel filter next... then maybe an IAC cleanup if that doesn't do the trick.

These cars are well engineered. This one has 182K miles on it and still drives and runs great. Usually catalytic converters don't just "go bad." The computer was making up for the vacuum leak by spraying more fuel to the cylinders to prevent a lean-out condition.

Hope this helps.
#240 - Vegas T-Bird - 02/23/2012 - 07:06
po420 on c. pacifica
#241 - kanika morgan - 03/06/2012 - 13:33
po420 code
i have the code po420 had it scanned and said
cat effeciency low bank 1 so my question is what one is it before or after the cat on bank 1 thanks
#242 - kevin - 03/08/2012 - 17:59
po420 code
sorry forgot this is on a 1999 gmc with the 5.3 thanks in advance
#243 - kevin - 03/08/2012 - 18:05
P0420 Code
Ok, so I have changed both O2 sensors and the check engine light came on and I still have the same code, is it safe to say that the converter is out and needs to be replaced? Please help!!
#244 - Chris Martinez - 03/08/2012 - 23:42
SES again every winter
I tried the injector fluid again, cleaned O2 sensor, and even a new gas cap on my 2005 altima 4 cyl. The SES/p0420 light stayed on until i pulled neg battery cable to reset computer.
#245 - Tedd B - 03/11/2012 - 00:34
I have 2002 Toyota Solaris 2.4 lt. Code P0420 is trouble code. What's wrong & how do I fix it.
#246 - paul - 03/11/2012 - 12:19
P0420 Code AGAIN!
have just replaced both O2 sensors and converter on my 01 civic ex. all was fine for about 2 months.
Now... p0420 code again. I was told that the factory cats are far superior than aftermarket and I may need to go the factory route. Quite pricey however for a can with 210,000 on the clock. Any thoughts??
#247 - Wayne - 04/09/2012 - 12:04
toyota echo
I have a 2000 Toyota Echo with 100K miles and p0420.
#248 - Screw - 04/15/2012 - 16:33
po420 cat effic below threshold bank 1
I have 1999 jeta with po420 cat effic threshold (bank 1) engine code and no pawer and knocking please send me and email thank you
#249 - 1999 jeta 6 cylender - 04/17/2012 - 16:27
Not a Computer
I have a 96 merc sable with a P0420 code which has been coming on every 30 miles or less for the past 5 yrs.Current mechanic cannot give me a definite answer what the problem is.I think he is just, well you get my point.
#250 - Dave B - 04/21/2012 - 03:54
loss of power in my 2006 kia rio
I have this p0420 code,but let me exsplainw,afew mothes bak I needed to replace the spark plugs and coils,but didnt have the $ so I continuded to drive the car for afew days then al of a sudden my car had a large amount of loss power and I got off the high,opened my hood and found my catalytic converter glowing orange red hot.i changed my spark plugs and coils and the car ran fine till bout a week ago,i have a loss of power going uphill,at redlights,and stop sights,when I excelorate,i cant max out my gears speeds,also at night the car seems to run better,i thaugjt it cat converter but theres no bad smell,no smell at all,and it hasnt heating up like befor temp gadge is at normal.sumone help plz,this car is how I survive.
#251 - Holly - 04/23/2012 - 14:22
P0420 code...
2000 Honda Civic EX..Not sure why my car would not start my husband changed my fuel pump and then turned out to need a new engnition coil. When it finally started it ran bad.. TO MUCH GAS..but got fine after running a while. Now my check engine with P0420 code. Can you help what we should do next? Thanks so much!!
#252 - Penny - 04/24/2012 - 14:05
I just bought a pilot 03 and i see the famous P0420 error code - Catalyst system Efficiency below threshold. The only thing I noticed was that the engine oil was getting low so I changed the engine oil and wiped the check engine sign. I do not know if I solved the problem. Any ideas!!!
#253 - Bee - 04/25/2012 - 14:53
catalyst converter can catch fire
The reason why your getting a P0420 and the reason why your radiator fans keep turning on is because your catalyst converter is getting very hot because it is clogged with half burned oil. Reason being if you have a 2004 chrysler town & country like i own or any other chrysler or any other make van or car and you are losing oil.Well that's what my mechanic told me today. Well I. brought my minivan to my mechanic today because my check engine light came on and temperature gauge was going up and up radiator fans did not want to shut off. Well first I changed two o2 sensors and my temperature sensor. My check engine light was still on so he asked me if my van uses alot of oil he said that's your problem he said the half burnt oil (sludge ) gets caught in your catalyst converter and your cat is clogged from this oil burning problem (Blow by).The catalyst could catch fire. So I suggest if your vechicle is going through alot of oil you should complain to the NHTSA so the more complaints the better. They will push your dealer for a recall
#254 - Frank - 05/09/2012 - 11:32
1997 Ranger Ext-Cab 2.3 two wheel drive w/ A/C
Just wanted to let folks know that it seems like I solved my P0420 code. I read all of these posts and was confident my catalytic converter did not need to be changed contrary to a few mechanics knee jerk opinion.

This motor has 98,000 on it and is sporting a generic catalytic converter. That told me that the original one was replaced most likely because the P0420 code was previously diagnosed incorrectly as a bad catalytic coverter. No other codes were being stored in the computer's memory.

My solution was to do a tune-up and look for exhaust leaks before the converter and intake leaks before the motor. I found no leaks. This particular motor does not have the ability to adjust the timing so that was not considered. I determined that the next thing would be to ensure the up-stream oxygen sensor was working properly.

I did not have an oscyliscope to compare the up-stream sensor to the down-stream sensor nor an advanced OBDII scanner to pull fuel trim and real time information on the sensors. What I did do was buy a $19 up-stream oxygen sensor off eBay and put it in.

Replacing the up-stream oxygen sensor has solved my particular P0420 check engine code. I wanted to let people know this to encourage them that it is not always the catalytic converter that is the problem. It may only be the symptom of a poorly running motor.

To fix your particular P0420 issue I encourage you to make sure the engine is performing perfectly, sensors and all before spending the money to replace the catalytic converter. Converters are killed by poor running engines. Cheers and good luck.
#255 - Reben Mitchell - 05/15/2012 - 14:07
'00 Odyssey P420 came again
It is frustrating to see what do these auto repair shop mechanics do to fix the root cause of the problem!
I got the P420 code and then transmission related codes.. I have changed CC, O2sensors, EGR valve and changed the Tranny with Re-manufactured one.
After 2K miles, P420 showed up again. Now it is frustrating to even think of dumping another $K (mostly on labor) to fix this old crap.
#256 - Boby - 05/16/2012 - 15:24
Injector cleaner was a temp fix on my 2005 Altima S AT 130k. We baby this car and well maintained. it seems the Nissan P0420 code overrides other codes. This maybe why no crankshaft code, but the fix so far is the crankshaft sensor!
#257 - Tedd - 05/26/2012 - 10:34
po 420
My wife's 98 grand am 3.1 has had the po420 code for about 6 months. About a year ago I replaced the converter, both 02 sensors, coil packs, fuel pump and a total tune up, plugs, wires, air etc.... Everything was fine for about 6 months then the light came back on and now recently her gas tank thumps about 20-30 seconds after driving it with a fuel smell. Visually everything looks good with no leaks. Is this an evap problem?, is this why my po420 light keeps coming on? Any help or feedback would be great!
#258 - Don Clenton - 05/27/2012 - 17:18
Inaccurate information.
I disagree with the statement, " If the reading fluctuates then the sensor is likely bad and will need to be replaced"quot;, when performing diagnostics for a p0420 dtc, monitor your o2 sensor voltages pre cat and post cat, bank 1 sensor 1 pre cat should be switching quickly, post cat bank 1 sensor 2 should be moving, but slowly, if both are switching quickly, this would be caused by a degraded catalitc converter allow exhaust gases to exit quickly, not a faulty o2 sensor. If the cat was clogged you will most likely have misfire p0300 dtc's and a insuffient activity code for bank 1 sensor 2.
#259 - Rich - 05/29/2012 - 19:15
P0420 code
I just got my 1999 Dodge Caravan returned to me with the check engine light on and I took it and had it checked out and it came up the code P0420. Does anyone have a first suggestions on what I can do first.
#262 - Diane - 06/11/2012 - 11:56
I have a 2004 Saturn Vue and failed emissions with P0420 and P0230 codes. What should I do first. I hope I don't have to get a Catalytic Converter.
#263 - Sherry - 06/20/2012 - 09:43
2004 chrysler 300m
I have a 300m with 3.5 v6 po420 code came up I just had new cats o2 sensor put on just a little over a year a go I was having cam sensor prolem what going on ??????
#264 - VIC - 07/14/2012 - 21:57
I have a FORD EXPEDITION 2007 5.0 V8 the problem is CATLYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1) What i have to buy to fix my truck??????
#265 - Xavier Morales - 07/20/2012 - 04:29
i have a 2005 toyota corolla the PO420 CATLYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1) came on i changed the before catalyst sensor the check engine light came back on, what else can i do, thanks
#266 - Joel - 07/22/2012 - 13:40
P0420 idea from 18 yrs of experience
As a mechanic with 18 yrs of experience working on weed eaters to bulldozers (it's easier to state what I've never worked on, airplane, helicopters and trains). Today's vehicles are not like the ones of yesterday, when you could just state the symptoms and noises your car was making to the mechanic, then he would fix it.

I'm sorry not that easy. As stated earlier the dtc simply is tell you that the control module (cm, ie computer) thinks the pre cat o2 sensor and the post cat o2 sensor readings are near the same, to the cm it means the vehicle is not running efficiently (creating excess pollution) . Which can be caused by a number of issues. All sensors whether pressure, position, or temperature adjust voltage of the circuit which is then interpreted by the CM.

There is no clear answer to what needs to to repaired without further diagnostics. Every part related needs to be checked. The repair could be costly (cat replacement) or very cheap (cleaning a connectors contacts). The Cat might be in need of replacement, one of the O2 sensors may need to be replaced, the CM could be faulty, a wire in the circuit maybe chaffed and touching a power source or a ground source, or one of the connectors maybe corroded.

In the short, there is two ways to repair the vehicle, one can be very costly (the wrong way, although you might get lucky) the other can be very time consuming.

1. Pick a part, purchase it, replace it, and test drive the vehicle. If the light goes away, woo hoo it's fixed. If the light is still on pick another part and repeat and keep repeating till the light goes out. This method could result in replacing all electrical and mechanical parts.

2. Properly diagnosis the all items related to the system. Which may include the use of scan tools, digital multi-meter, access to the proper repair information, and other tools and test equipment. Take the time and do it right. If you do not have the skills and knowledge for this method, find a good repair shop.

A little secret to finding a good repair shop is to look around the shop and ask questions. A shop where you have to set an appointment is likely to be a good place, it is likely to be busy because they do good work. Ask them what type of diagnostic equipment do they use, where do the obtain their service information, how long have they been in business, will they offer a warranty, etc. The less they advertise and the larger long term return customer base the better the shop.

The best answer I have to all these questions is properly diagnosis the system and repair the minimal amount of items to achieve success.
#267 - Jesse - 08/16/2012 - 17:39
2006 jeep grand cherokee code 0420 after engine misfire
Hi All-
This past weekend I thought I would do a nice thing and clean and Detail my wifes car. I cleaned the engine as I have done numerous times before with Simple Green, taking care not to get the electrics wet.
Well the next day it started bucking, hesitating and tripped the MIL. Scanned it, found a random misfire. Pulled the coils, found moisture on 4 of them. Blew them off, sprayed with WD40, and blew off again. Blew out the wells where the plugs are, found a lot of water in # 2. put back together, still had a slight skip, cleared codes, reset the MIL.
Wife drove it the next day, a lot of hesitation and the mil came back on. Scanned and found # 5 cyl misfire. Pulled all coils, #5 had a little moisture on it, the rest dry. Replaced all plugs, blew out all wells again, reinstalled, reset MIL, took for a long ride, ran great!! nice and smooth.
Wife drove it again, MIL came back on, rescanned, got a code 0420- Catalyst System Low Efficiency. I only drove the car about 50 miles with the misfire.
Are the cat's that delicate that it could be ruined, or maybe a o2 sensor?
Does anyone have any Ideas?
Thank You.

#268 - mike sellitto - 08/31/2012 - 21:03
Recently i have had the check engine light come on in my car. I took the car to the dealer and they replaced the cataylated converter. with in 50 miles the check engine light came back on. I took it back to the dealer and they replaced the 02 censor. which i have already replaced 3 times and replaced the front o2 censor and the coolant censor. with in 150 miles the check engine light has come on again. this process has taken 2 months and i am getting very frustrated. spending time and money for these repairs. Can anyone fix this car correctly so i don't have to buy a new car. now is not the time for that in this economy.

I appreciate any help anyone can give me.
#269 - Robin - 09/02/2012 - 12:51
#270 - armada - 09/02/2012 - 15:59
P0420 2001 Pontiac Grand Prix
i have practically a whole new exhaust system, just replaced my upstream O2 sensor and my check engine light is still coming on with P0420 as the problem what else can it be thats causing this.
#271 - Norman - 09/12/2012 - 10:21
Ive replaced all catalytic converters as of last yr.including oxygen sensors.codeP0420 came up twice.what else would trigger? Car is a 2004 Ford Crown Victoria Police Interceptor
#272 - Mike - 09/21/2012 - 10:52
I have a 1999 Infiniti QX4,engine light on,overall test result:failed and shows codes P0420 and P0430 Catalyst System Efficienc,Bellow Threshold...what thats means,how many catalyst this car have? Can you help what we should do next? and where are those located? thanks for any help!!
#273 - Oscar Roman - 09/26/2012 - 16:11
p0420 repair
great posts by the techs only proper diagonosis fixes cars really like laquar thinner trick though also tide in a bucket to clean cat
#274 - john - 10/08/2012 - 17:00
I have a 2011 Ford Fiesta. After code I replaced spark plugs, air filter, downstream and upstream O2 sensors. OBDII reader shows cat inlet and outlet temperatures differ by 175 degrees and all other readings are good. Light only comes on about every 750 miles and only when pedal is between acceleration and deceleration. Car has 215,000 miles on it (215,000 is not a typo).
#275 - Mike - 10/14/2012 - 13:47
I have 2003 Chevy avalanche with P0420 code and the motor is running rough shouod I change the rear o2 sensor or the cat
#276 - Andrew swanson - 10/14/2012 - 15:52
P0420 Code
I have a 2006 Ridgeline that is showing this code. This code first appeared about two months ago and I reset the computer only to have it reappear again. The vehicle only has 57k miles. I have done a visual inspection and can see no problems. The vehicle is running fine. What should I do?
#277 - Steve - 10/19/2012 - 10:48
Lights out!
I have a 2003 Ford Expedition and have the p0420 Catalyst System Efficiency Below Threshold[Bank 1][5]showing. What does this mean? Thank you for your time and help.
#278 - Francisca - 11/02/2012 - 01:57
My engine light is on and the mechanic told me that it was my Catalyct Converter. 2003 Dodge Neon.
#279 - angela aaron - 11/02/2012 - 21:03
cat converters are designed to last for 10 yrs unless contaminated by engine most often this code is set by exh leak even if very small also o2 sensor can be going bad if your car needs a tune up it will also affect the o2 sensor readings to view o2 sensor activity youd need a real scan tool not a code reader from autozone
#280 - joe - 11/05/2012 - 20:19
p0420 code for 2008 nissan versa
I see that if you have the p0420 code showing that it can be the rear 02 sensor or cat converter. However can it also be the upper 02 sensor which is also called the air fuel sensor???
#281 - Pete - 11/15/2012 - 13:58
PO420 code with strong fuel odor in cabin.
2003 Toyota Highlander.
Trying to keep up with 4 cars is not fun! lol Hope this doesn't get too expensive, funds are depleted.
Thanks ahead!
#282 - Rick - 11/19/2012 - 21:18
on toyota corrola 03 how easy is to replace the rear 02 sensor?I cant realy see the plug head,it seems to go inside of a bi plastic encloure.
#283 - mike - 11/27/2012 - 14:36
i have toyota tacoma 2006 and have problem by code p0420, may i have the picture of the part that has problem
#284 - magi - 12/10/2012 - 21:02
Toyota 4 runner 01
had this before P0420 replaced the plugs and i ran a half of a quart of xmsn fluid in the fuel tank to clean the injectors toy has 200+ thousand worried tho that the timing chain is gettin loose w the miles on her tho or will punt and change the cat sensors and go with a non cal cat
#285 - tom mccrary - 12/14/2012 - 21:19
GREAT INFO! But, are the comments for real?
Did you all read the website above? Every last one of you didnt do everything on the list, and then say how do i fix it now? What do i do next? lol Please tell me its a joke!
#286 - Anthony - 12/19/2012 - 21:11
2007 acura mdx
I bought my 07 Acura MDX about a year ago with 11000miles and check emission light on. It is now on 15000 miles and the last mechanic visited read the code and said catalytic efficiency less than 0.6 What does this meant and what should I do?

#287 - Komolafe, Sunday Olanrewaju - 12/28/2012 - 15:35
code po420
code po420 après remap ;subaru wrx 2004, cata sport et remap stage2
#288 - darlis - 12/30/2012 - 12:48
I was thinking to by pass O2 sensor,but I saw to move away the sensor to change readings and solve problem.
#290 - Bogdan Odwazny - 01/04/2013 - 23:39
This fixed my PO420 code
2008 Nissan Maxima 66,000 miles
The MIL light came on one morning, right after it was driven for a while, stopped and restarted at a restaurant. I was traveling, the car was running fine, no work had been done to the car, the milage was low, thought it was likely the loose gas cap problem. The MIL went off in a few days/several driving cycles. Two days later it was back on. Autozone got PO420 code. (Never got a chance to check the first code).
I new the air filter was "dirty" but it did not really look all that bad but I was planning to change it at the next oil change anyway so I put in a new A/C, then filled the 1/2 tank with BP premium (Invigorate stuff). The light stayed on a couple of more days, on the next long leg of the trip I put in another 1/2 tank of BP premium. Near the end of the day, on another food stop, the MIL light was off when I restarted the engine. It has been off over a week.

I am not promoting BP, just relating what I did. Truthfully, I think it was the A/C, it may have been the OEM filter (I bought the car used a few months ago)

Hope this helps someone. I learned a lot reading through this forum.

Good Luck
#291 - RMay - 01/06/2013 - 18:12
I have a 2005 Chrysler Pacifica and failed an emission test due the error P0420 Bank 1. The cat converter and exhaust were replaced by the dealer in 2010 but the check engine light has been coming on sparingly for over a year. I just recently started smelling gas.
#292 - Trent - 01/10/2013 - 09:35
I have Nissan sentra 2001 GXE California
new catalyst have been in place then after 600. mils light Cod PO420:and P0430 com on Catalyst System Efficiency Below Threshold (Bank 1 & 2)
Do you have comet advice what to do
#293 - Sam - 01/15/2013 - 01:37
PO420 - Catalyst System Low Efficiency
Dear All,

I have Chevrolet Aveo 2007. From few days I saw this Yellow light.I did Computer scaning inspection they said thatPO420 - Catalyst System Low Efficiency. I'm still driving the car there will be any problem, Please advice me what to do
#294 - Shoaib - 01/15/2013 - 05:32
Dear Mr. something to do with cam sensor,may be you need new one,Looks bad smoke could affect O2 sensor and shows PO420
#295 - Bogdan Odwazny - 01/15/2013 - 20:13
impressed! I`m having trouble with dtc p1416 in a 1998 hyundai accent . Help
#296 - main mann - 01/23/2013 - 09:47
p0420 code
I have a 2003 honda crv ex i have change the upper oxigen sensor with a brand denso but the down oxigen sensor with a different cheaper brand the engine light went off for a full month without turning on. then it went on again ask the honda tech about the oxigen sonsor they said honda will recognize denso but not bosh or any other brand. I have also notice that when I put gas and drive a few miles the light goes on again then after three different drives it goes off. then a few mile on different days it goes on again. should i change the cheap sensor to a denso or should i change the cat.
#297 - Lissette - 01/24/2013 - 01:26
Today I turned on my car in the morning, after a few minutes i noticed that the check engine light was still lit, i went to my job when i get home the curios little light was turned on. that was when I decided to buy the ideal software for my car which is a 2001 Mazda Protege it cost me $80 box i install in my laptop the interface cost me $100 now I am looking for information since my car has a this DTC P0420 I will let you know what going on whit my curios light.
#298 - Israel - 01/29/2013 - 20:17
Lacquer thinner?
Lacquer thinner? Has anyone tried the lacquer thinner besides Bob and had it work. Bob I'd love to hear if it worked long term for you. I'm about to give it a go and am super curious about its results.

#299 - drew - 03/04/2013 - 10:10
#300 - william - 03/11/2013 - 20:54
Fuel Injector Cleaning
@ Mechanic,

Fuel injector can be cleaned with fuel injector cleaner, duh. Go to any auto parts store (even walmart), and buy a bottle of "Lucas tune up in a bottle".
#301 - Eric - 03/21/2013 - 22:10
2002 Chevy Avalanche 133,000 miles.CEL one month prior to inspection time (of course).Codes PO134-PO135-PO155-PO420.Replaced bank 1 and 2 forward O2 sensors,light still on.Replaced intake gaskets per GM service bulletin.Light still on.Found bad O2 sensor fuses,replaced,CEL still on but now have PO420 only.After reading everything here,don't know what to do next.Clean MAP?Replace PCV(does it have one)?Tuneup?Check/replace rear O2 sensors?Check for exhaust leaks?Printed a list of all these options.Taking to my mechanic who still has my truck.Check back later.Open for suggestions! Anyone? Thanks all !!!
#302 - PO420HATER - 03/22/2013 - 14:46
Had the same code in 2001 Corolla. 3 gallons gas and 1 pint laquer thinner fixed it. I ran engine for 20 minutes at 2500 rpm and drove out on the highway. Alot of smoke at first but now engine light is off! Key is to run it through the tank quickly and not let it sit in there.
#303 - Jon B - 03/30/2013 - 17:31
I have a 2008 neisson verser and engine light is on.I ran dio check and code 0420 came up.I was told that cat-cvert needs to be replace.Is this correct?
#304 - joseph taylor - 04/08/2013 - 17:26
Code appeared on my 2001 Tacoma 0420 and I took it to my mechanic ,same story as the rest of you guys. Need cats and after that o2 sensor. Check engine light kept coming on. Tried after reading tips and put in sea foam and 93 octane. 93 octane I tried first still had ethanol. No change. Check engine still came on. Looked on line to see who in my area sold 100% gas. Found one location a bp right down from house. 89 octane. Cleared code and light came back on in 30 miles. When I got to 3/4 tank light went off all by itself. Let you know if it comes back on. Thanks!!!!
#305 - Good old boy from Fayetteville,n.c. - 04/09/2013 - 16:00
Light still not coming on my 2001 Tacoma. Lots of trailer pulling the last few days. Running ac as I pull. Now have over 150 miles since I put in bp 89 octane 100% real gas. Feeling pretty good. Keep you updated.
#306 - Good old boy from Fayetteville,n.c. - 04/10/2013 - 22:34
Engine check light on with error code P0420
Dear Sir ,
Recently the engine check light have been on.The diag engine program show the P0420 error. I have done the below actions:
1- changed the front and behind catalyst O2 sensors.
2- Cleaned(washed) the fuel injectors.
3- Control and check the exhaust connections for leaking
4- And finally replaced the catalyst

But unfortunately the engine check light in on and the P0420 is appear yet.
What can I do for solution this error?
Thank you for your attention.
#307 - H.Soleimani - 05/04/2013 - 06:24
Catalyst Efficiency low-bank 1
I live in a county in Ohio that requires an emmissions inspection. The check engine light is on solid and I have the P0420 Code. The technician that services my car looked at the vehicle and unplugged the battery and reset the codes, The emmisions inspectors said I could get a old-fashioned tailpipe reading. The P0420 code is the only code showing. What is the likelihood that I will pass the inspection with a tailpipe inspection with problems related to that code.
#308 - Robert Charles Gribble - 05/04/2013 - 16:12
Toyota Solara 2004 4 cyl 158K miles P0420
Flex fitting was leaking on frt exh pipe- replace pipe with integral cat and secondary o2 sensor. Still getting a p0420. Car running great. Is the next step the front cat/manifold? Is there a separate code for the o2 mix sensors on the exh manifold or do you need a scope to determine proper operation of sensor?
#309 - Jeff - 05/26/2013 - 11:15
Code PO420
My check engine light is on. I took the car to Advance Auto the code PO420 is coming up. Whenever I cut my car off to pump gas the car keeps cutting off. I have to keep my foot on the gas for a couple minutes then it stop cutting off. I have no more problems until I put gas in the car again, but the check engine light says on is this related to this code.
#310 - LaPrecious - 05/28/2013 - 14:11
Subaru 150,000 miles
I have a PO 420 code my mechanic says it the Cat because the readings are the same on both o2s is this right?
#311 - Dave - 05/28/2013 - 17:28
automobile engineering
please what can cause a toyota camry to be start and off immediatly
#312 - Adebayo-muritala - 06/01/2013 - 17:55
2001 Subaru Forester
I am getting codes p0301 & p0420. The car has 160,000 miles. It runs slightly rough at idle, and idling around 350 to 450 rpm.
Other than that I dont notice any difference. Im out of ideas.
#313 - Shawn Hon - 06/04/2013 - 08:47
1997 Toyota camry
Car runs great but I warm it up in the morning if it is hot early I turn the air I see the temperature gauge drop from being in the middle of the hot an cold to almost cold bottom an slowly but surely or never goes back to the middle an just today I got a check engine two Weeks ago my mechanic said fill up on 91 an I have been doing it ever since been great I dont know tomorrow I will see what the modus or whatever the snap on tools name is theh use at his shop. Any suggestions he cleared the code last time but it was p0420 bank 1 he said o2 sensors look good he said 9/10 times the camrys front catalytic converters goes out before the rear one I checked into that very expensive so any help im going to replace it but any suggestions before I spend few hundred
#314 - John wayne - 06/14/2013 - 01:57
I have 2003 Subaru outback legacy wagon with a p0420 code..u mention to do a ECM reflash...can u explain what that is...thanx
#315 - william - 07/03/2013 - 16:58
Mechanical Technician
I have 2007 Hyundai Santa Fe 2.7 lt. Code P0420 is trouble code. What's wrong & how do I fix it.
#316 - Gonzalo Gonzales - 07/08/2013 - 08:16
van runs good ..just all sudden wont go fast just creeps along
#317 - bash1 - 07/19/2013 - 19:05
2002 Suzuki XL-7 2.7L
Sorry for my English
I bought this SUV with 100K miles. Use it for about 3 month, then Check engine light is on, scanner said code P0420.
The first thing that I did was check spark plugs. They have a normal appearence, but aren't that user manual recomended.
The previus owner put spark plugs (regular platinum)of $3.
The manual recomended ones that costs $13 each. I replace them and put NGK Lazer Iridium spark plugs.
I Erased check engine light, it never goes back.
Mi little SUV runs better now
#318 - Mac - 07/23/2013 - 22:18
I have a 2002 Camaro v6 with P0420 code. Scanner says low efficiency bank 1. I replaced the o2 sensor after the cat in case it was reporting back the wrong info. Should I have replaced the one at the driver's side manifold since it says bank 1? If it were the o2 sensor on the passenger side wouldn't it read P0430?

Thanks for your help!
#319 - rod - 08/01/2013 - 09:35
Codes in order:
P0420 Eng. $10 Catalyst System Efficiency Below Threshold Bank 1 (error code twice)

P0420 Mod $10 Catalyst System Efficiency Below Threshold Bank 1 (confirmed)

Check Engine Light

How do you determine if the Catalytic Convertor(s)are bad?

2004 Toyota Solara V6, 115K miles

#320 - Brad Still - 08/02/2013 - 13:05
2003 buick park ave ultra
Have the p0420 issue. I've changed the cat converter, plugs and wires. I found out the the intake manifold gasket is leaking coolant. Could this cause the error code?
#321 - Rob - 08/06/2013 - 09:32
PO420 & Smell of Fuel on Hot Day
My '95 Toyota Landcruiser is OBD2-compliant and reports the PO420 code. Shortly after the MIL came on, I noticed the smell of gas. It was a hot day over 100 degrees. I was 4-wheeling and likely sloshing the gas in the tank a bit. When I removed the gas cap, there was a rush of pressure released. The pressure release had been noticed previously at fill-ups but not this extreme. I replaced the cap but the pressure is still there. I have read that a charcoal evaporator under the hood can get saturated with age/miles and be a culprit.
The truck is new to me and I had to replace the double CATs to pass smog in CA. It has the proper pair of aftermarket Magnaflow CATs. O2 sensors are new. What do you suggest?
#322 - Don Larson - 08/09/2013 - 10:27
2000 Potiac Bonneville SLE Service Code P0420
I was driving home today and the service light has come on suddenly.I have not had a loss in power or anything the only noticeable difference is it has hesitated to start a little bit. Any ideas?
#323 - James Patterson - 08/16/2013 - 11:31
07 Pontiac g6
2007 Pontiac G6 with P0420 code @ 80,000 miles
Replaced O2 upstream sensor - same error code
Had a mechaniac diaganose that we had a blocked cat convertor.

Just found out - GM will cover all repairs on a bad cat converter up to 120,000 miles or 2017.
Just got the car out of the GM dealer - fixed and no charge.
Wished I had taken it to the dealer 1st - I would have saved the 02 and diaganose cost.
#324 - Gene - 08/17/2013 - 14:17
I,have a 2003 jeep liberty.i,filled up with gas during astorm.drove 5 miles and engine light came on.i,had it checked and the number po420 came up. the man at the shop cleared the computer and the light went out. my battery post is in bad shape, could this be a proplem with the light? tex gill Ozark al.
#325 - tex gill - 08/19/2013 - 13:22
ASAP Service
What do i need to do to fix a friends 2001 Dodge Durango that is code P0037?
#326 - chris leach - 09/07/2013 - 21:39
Writer of article
It is very obvious by all the questions here that haven't even been acknowledge (totally ignored)that the writer of this article isn't serious about helping anymore than what he's given here in the above article. There are other articles written by more serious experts who are willing to take the time and answer your questions. Besides, this article isn't the best or most precise anyway.
#327 - Karen - 09/17/2013 - 11:01
Giving P0420 does'nt always ask us to change the catalytic converter
My case is exactly same with above #318 post. When I buy my corolla NZ121 it was giving the P0420 code(Check engine light was ON). I took the car to SL toyota agent, they asked me to replace the catalytic converter as a fix. But fortunately I didn't accept their decision because that was a huge expense.
Meanwhile I noticed since the first day of buying the car that the engine knocks at the pick up time, So I checked my plugs, at that time I figured out that these are not the plugs which are recommended (brand NGK but not the recommended model) by the manufacture by referring my car user manual. I bought a new set of DENSO plugs as per the user manual recommendation from the toyota lanka(SL Agent). When I compare the older plugs with new ones they were bit shorter than new ones (this may cause for a firing point change in combustion chamber which leads to a terrific engine knock even if these plugs are in good condition). Any way I replaced old set with new set and cleared the P0420 error code by using the scanner...Afterwords I made a considerable mileage So far I never see the check engine warning again :D
#328 - Buddhika Wathukarage - 09/24/2013 - 15:01
catalytic converter
my truck is a 2005 expedition.i have the P0420 code and am sure it is the catalytic converter. the truck was running fine but just loud and now all of a sudden it starts running rough after the engine is warm. like it is going to stall. this code came up and also the same one with $07E0 and also then $07E0Pd any ideas?
#329 - vanillla - 09/29/2013 - 10:03
Clean the MAF Sensor
When ever I get the P0240 code I just take out the Mass Air Flow Sensor, clean it with electronics cleaner and pop it back it. My Tacoma (2001, 187000 miles) is then good for 6-8 months before I have to do this again. My truck's an Phoenix Arizona truck so the air flow sensor tends to get really dirty often. For the Tacoma the MAF Sensor is an $85 part so it worth trying cleaning it with $5 electrontic cleaner first.
#330 - Dot Matrix - 10/10/2013 - 12:33
P0420 Pd P0420 ??????
#331 - Mark - 10/14/2013 - 04:17
Code P0420 CAT
My Camry 2002 4Cyl. ECL was on found code P0420. said "Catalyst System efficiency Below Threshold (bank 1)
How Do I know which CAT? ( Downstream or upstream failed)?
or O2 Sensor issue? I had replace new Bosch #13733 O2 sensor (upstream)... ECL is still came back after 100miles
#332 - Mixer - 10/16/2013 - 20:25
ERRORE P 0420 E 0430
#333 - GIOVANNI - 11/07/2013 - 13:28
p0420 code
alright so have changed both o2 sensors and put a magnaflo(high flo)cat. when changing the o2 sensor in the manifold it had stripped so i welded a piece on top.. now im assuming that the o2 sensor in not in far enough to get a good reading and that is why i am getting this Catalyst System Efficiency Below Threshold (Bank 1)

thanks for the help
#334 - jason - 11/14/2013 - 12:54
car repair
Code p 0420 replaced both o2 sensors still check engine came on.2002 chys town country engine temp ok catalic converter not getting hot,
#336 - Marty - 11/23/2013 - 19:12
automotively disabled
Have 05 toyota matrix ck engine light came had it diagnosed! Then mechanic told me it was upstream 02 sensor1! Had it put on light wont go away runs worse than ever! HO HUM!!!!
The code diagnostic machine pulled was P0171!!I live on 600$ a month. People how much you make so i cant keep trying different fixes on my car!! I truly need help
#337 - Judi Sims - 11/24/2013 - 09:06
Nissan Altima 2005
Catalyst system efficiency below threshold- bank 1 probable cause
Large vacuum leak
fuel system fault
ignition system fault
failed bank 1 catalytic converter
#338 - Daniel - 11/25/2013 - 08:35
There was a crack in the flange on the left bank of my 2002 5.7 Ram Air. I had someone weld the flange at the cat convertor.I drove about 100 miles and the lite came on with the P0420 code. Will welding on the cat ruin it?
#339 - Stan Strack - 12/06/2013 - 19:13
I have 2008 Hyundai Entorage and having engine light on with the code P420 and vibrating while driving sometime and hard to go on hill
Please let me know ASAP.
Thank you and have a great day!
#340 - Me - 12/08/2013 - 16:52
Dodge Ram 1500 5.9L Crew Cab RWD. After giving a tune-up(replaced wires, spark plugs, cap and rotor)to get rid of P0150 codes, everything was OK for a month until 2 day ago. P0420 Code.
#341 - Vieques - 12/21/2013 - 12:18
I am having below codes on my 2003 sienna from 10,000 miles now but it does not give me any problem though its check engine light is on
Sienna fault codes:
P0171 Bank1 system too lean
P0174 Bank2 system too lean confirmed
P0420 Catalyst System Efficiency Below Threshold Bank 1 confirmed
P0171 Bank1 system too lean pending
#342 - Frank - 12/21/2013 - 22:58
I have a 2001 chevy s-10. the truck runs rough until it warms up winter or summer. I replaced the IAT sensor,The MAP sensor. The IAC valve is next. the code keeps reading 420. Maybe its the oyxgen sensor? I think it is a temp related issue as it runs fine after it warms up. Mechanic said the cat is working fine on it. I do hear a whistling noise when i first start out when its cold out then it goes away. Dammed if i know what it is. i have replaced just about every vacuum line on it. Gas milage is terrible on it now
#343 - allen hall - 12/24/2013 - 08:34
When the car is idling the car revs up and down till the car quits running. p0420 came up as the code.
#344 - Steve - 12/27/2013 - 13:29
I have an 2001 L200 saturn and I replaced the O2 sensor but now my car feels like it is misfiring any suggestions?
#345 - Jackie - 12/27/2013 - 15:18
I'm beating my head against the ground with this p0420 on my 99' Honda crv. It started with a rattling cat and a few days later the check engine light came on with the p0420. Since then in order I replaced the cat, both O2 sensors, fuel filter, used fuel injector cleaner in the tank, cleaned and regapped plugs and last I thought for sure would work I used the plug defowler trick on the aft 02 sensor. Now I'm not sure how the sensors communicate but exhaust leak my set off a code even with the aft sensor backed out of the pipe with the defowler. Any advice would be helpful. As I remember the cat installation forward bolts have springs that keep it tight. Maybe I need to torque them down more. Thx
#346 - sean - 01/01/2014 - 19:38
for the people who replaced cat living in CA
For those of you who have replaced their cats and all this other and still get the code. if you live in CA you must use the OEM cat and generally its a dealer only item. some vehicles may use an aftermarket cat. check here to see if you can use an aftermarket cat. the cat you have put on your car may not be up to par with California emission standards, so it throws the code. not sure if CA registered cars have the tighter standards programed into the ECU or not. but I know when I bought a universal cat I kept getting the code... I then bowed down to the stealership and paid an arm and a leg for a stock cat and all is good now.
#347 - Michael - 01/07/2014 - 16:53
Glorified DIY
Thx for the help on this guys!! This PO420 code came up on the wife's '10 Camry. After Advance cross refrenced the codes possibilites, the diagnosis appeared to be the rear O2. Brass bristle brushed, steel wooled and soaked in gas. I also cleaned the MAF w/MAF cleaner. I then powered down the battery and 500 miles in, still no CEL...sooooooooo simple, even for me!
#348 - Led - 01/08/2014 - 09:27
Thank Eric!
i have a mazda protege 2000
I changed the pcv valve, spark plug and wires
now 3 weeks after no more po420 code
good luck to all!
#349 - henri - 01/11/2014 - 08:29
After Misfires, fixed plugs, coils, now 0420
My car: 2002 altima, 94500 miles, 2.5SL

Around 6 months back, my alternator went bad. And since then it has been struggle with my car.

1) Replaced alternator with after market one. That went bad in 2 weeks. Then bought a used alternator(but original). Things have been fine with alternator since then.

2) After few weeks, Engine Misfires started(on particular cylinders). Flashing CEL sometimes. Replaced 2 ignition coils and all spark plugs after repeated misfires.

3) Never had engine misfire while i am driving. Only rough idle in drive(when stopped at traffic light for example). as if engine is gasping and does not enough to breathe. Will stall.
But no such drama when driving on highway.
EXCEPT that i feel there is less power as i accelerate, Fuel efficiency has decreased.
And yesterday, my CEL light came on and code said 0420.

My dealer says catalyst converter needs to be replaced. I feel that it is a vacuum leak(intake manifold). I am a beginner so don't know if my hunch is right. Autozone says that it could be oxygen sensor also if not catalytic converter.

I live in north New Jersey and if you know a good engine mechanic, please let me know. Don't want to spend money on speculation.

kindly help.

#350 - Nitin Khosla - 01/12/2014 - 18:35
I have a 2001 Camary 4cly put a new cat on and still have a 420 code any sugestions
#351 - mike - 01/25/2014 - 13:17
I have this code on my 04 a audi and I was reading up on the page n someone said it can be the catalytic converter if u could hear a rattle inside the car and I think my car does have a rattle but it's hard to tell where is comin from and I think it could be my catalystic converter and I also got the code P0441
#352 - Chris - 02/04/2014 - 00:44
po420 code cause

#353 - nick - 02/10/2014 - 13:54
New Gas
10%+ Ethanol is causing the problem, The Obama Administration wants to increase to 15 to 20%, then there is the E85 which is worthless.
#354 - Cleveland - 02/11/2014 - 12:48
Kia Amanti 2004, Cat Conv "Po420" light is on.
Kia Amanti 2004:
After I changed the Spark plugs and wires, the next day I got po420.
I can hear kind of a rattle when the engin starts and still cold.
Cat conv are pretty expensive and I'm trying to check other posibilities causing the problem before I have to change the Cat.
Should I change all the Oxygen sensors?
Any advice please?
#355 - Fred - 03/03/2014 - 14:29
2004 toyota sienna
hello i am having trouble with my van and the codes are P2241,B2799,P0420 please help i just bought this van . i dont know what to do. thanks
#356 - c hristina - 03/04/2014 - 10:06
P0420 1999 GMC Jimmy
Had the p0420 trigger my check engine lift in my Jimmy. I figured o2 sensors so I got a good deal and replaced them all I reset the computer by taking the negative cable off the battery for about ten kin. Drove around for ten minutes and it came back on.. Any ideas what could be triggering my Service Engine Soon light?
#357 - Dustin - 03/07/2014 - 20:44
1996 Honda Civic - CEL comes on Periodically
Just to see if the CEL was a fleeting issue, I disconnected the battery and the light was not on ( I understand it was still in the computer). I drove a few days and 125 miles and the CEL came back on. Replaced fuel filter, another 100+ miles and the light came back on. It doesn't seem to be a continuous issue. Thoughts?
#358 - Gary - 03/14/2014 - 10:26
2006 Nissan Altima trouble with P0420 code
Any indication on the most common occurence for this code with Nissan altima's? i have ~ 145,000 miles
#359 - Jeff - 03/18/2014 - 13:59
I have a 1999 ford e150 base van 5.4 motor and I want to know we\'re should I start first with parts.
#360 - Tj - 03/20/2014 - 00:35
1996 Camry with P0420 code
I had the battery replaced and the engine light with the P0420 code never came back on. Simple solution.
#361 - Ruby - 03/28/2014 - 23:00
Don't blame the converter
I am not a mechanic but I help my friends and family with their cars, especially smog. I have cured about half a dozen P0420s and have never replaced a catalytic converter (except a melted one). P0420 may never be caused by a bad converter, yet replacing the converter with an OEM model will almost always clear the code. The new one is just enough better to bring the performance in line. A couple years later the underlying condition gets worse and the code returns. At that point a new converter may fix the code for a few months or it might not help enough at all.
P0420 is produced by the ECU when the downstream O2 sensor (after the converter) reports a high and fluctuating level of free oxygen where the level should be low and fairly steady. When we substitute "Excess free oxygen in exhaust" for "Catalyst system efficiency below threshold" the unimportance of the converter in the P0420 code becomes obvious. The ECU knows nothing about the catalyst efficiency, only about the oxygen in its output.
About half the causes of P0420 are exhaust system leaks forward of the catalytic converter. Vacuum leaks in some of the branches of the intake manifold, or leaks in all unequally, are the second in line. A poisoned upstream O2 sensor (silicone spray, I suspect) caused one. Dirty injectors contributed a few times.
Basically, anything that leaks oxygen uncontrollably into the converter input will produce P0420. Find it, fix it, and the code will go away.
#362 - Michael - 03/29/2014 - 17:05
Honda P0420
I am no pro but like to work on cars. This code has been a problem for about 2 years. This Honda V6 has 167,565 miles and runs very well mileage and performance wise. Lately it was giving me this code daily and recored every thing to see what was out of line. I notice my 02 sensors and Intake sensor were really not accurate but have been replaced as well as other parts. My brother told me to try electrical grease that is made for sensors to prevent any current from giving false readings. I did it and the check engine light has never lite up since.
#363 - Randy - 04/02/2014 - 13:05
2004 trailblazer codes/sensor verses cat
I have 2004 tailblazer and it has no power light, engine light is on, and codes p0106, p0132 and p0014 that is coming up. How do I know if a sensor is bad or if it is the CAT
#364 - tammy - 04/04/2014 - 00:26
Nissan Murano 2004 - 196.000 km.
After replacing 02 oxygen sensor error 0420 came after 40 km. Did not replace spark plugs yet; that's next step. Any suggestions?
#365 - Boslov - 04/05/2014 - 01:04
2003 Tacoma, 2.7L I4 gas engine, check engine light with P0420 and no other codes. I could clear the code, it would stay off for a few miles (up to 40 minutes of driving) and the light would come on again.

I added some catalytic converter cleaner ($25 a bottle at O\'Reilly\'s), followed the directions (add at 1/4 tank, drive 20 miles, fill up), cleared the code with my $15 OBD-2 reader, and 50 miles later the check engine light is still off. I\'ll update if it comes back on, but I\'m becoming optimistic.
#366 - RobertA - 04/18/2014 - 11:30
It didn't work: code was back on after 60 miles. Not saying the stuff doesn't work, but it didn't work for me.
#367 - RobertA - 04/20/2014 - 21:35
I found a leak on the gasket just upstream of my downstream sensor, and replaced this gasket (about $8, and an easy job other than working with rusty bolts). I cleared the code, and I'll report back if this didn't work.
#368 - RobertA - 04/27/2014 - 21:28
2004 Nissan Sentra P0420, P0420P
I have a Nissan Sentra 2004 American model (less than 100,000 km) that has seriously lost power, the OBD2 gives the above subject message. (P0420 catalyst efficiency below threshold (bank 1)

My mechanic said he would remove the catalyst convertor in front which he did. The car regained power but still not very good when i am on the highway.

he further says he will remove another one behind and see whether that solves the problem. Should I let him go ahead? what are the implications to my car's engine and performance if he does so

can I please get help me with some useful information?


#369 - Natfisa - 05/01/2014 - 05:56
code P0420 below threshold B1/observed 02 on 2002 subaru
How to fix it so check engine light goes off.
#370 - Bear Markham - 05/03/2014 - 11:36
I have a 2003 Chevy Malibu LS, and the check engine light came on. Got it checked, and it turns out to have a P0420 Bank 1. What would I do next, as I really don't what the check engine light to stay lit, as I could miss any other faults coming up.
#371 - Max Will - 05/13/2014 - 19:09
P0420 is on gf just bought a 2000 neon 204k miles.i think I hear a exhaust leak but the valve oil leakage is what's scaring me? All I see is questions but I need a answer
#372 - Dominic derenzo - 05/14/2014 - 21:56
Honda Acord 98 LX
My engine light just came on. I bought this car a few months ago from a used car dealership. I have keep oil in it regularly and even go to a great place that told me that nothing was wrong with it a few weeks ago they just recommended that I use high grade oil. Well I just had a ac converter (I think that was what you call it) put on and picked up today and my ac is finally working but on the way home today it had a engine light pop up. Nothing seemed wrong but this. What do you think is the problem?
#373 - Vicki - 05/23/2014 - 22:12
I have replaced the cat. convertor 3 times in 2 years, both sensors, changed to regular gas (no ethanol), reprogramed computer and still the engine light is on with a 420 code. Any suggestions?
#375 - mary - 05/24/2014 - 07:19
can u help me
My car is getting red hot under the bottom I am beingtold that that my catalytic converter is bad I need a new one they also say I need a tune up what can you tell me.
#376 - mike - 05/26/2014 - 17:52
2008 chevy 3.6
Have po420 code only
The purge solenoid. Was making now replaced light still on. Runs fine any help. Thanks Gary Austin Texas
#377 - gary - 05/29/2014 - 19:42
2002 volkswagen jetta
P0420 showed up when it was checked. My car overheats so much. Any help? Kemnacho Atl GA
#378 - kemnacho - 05/30/2014 - 18:08
2005 Chevy Cobalt
P0420 replace Cat, and rear o2. No exhaust leaks, no wiring issues.
#379 - Cole - 06/04/2014 - 11:04
Error light on Chevy malibu
My engine is skipping a little when I push on the gas. Now an error light is coming where my mileage should be. I have a 2006 model. Any ideas? I just serviced it. I thought I needed more transmission fluid, but they should have checked that when I got a full service oil change.
#380 - JANET SHAFER - 06/05/2014 - 15:13
audi A4 B8
i have changed the OEM exhaust with a new full exhaust system with a new HJS catalyst 200.

the new catalyst has been moved after the downpipe.

immediatelly i had the cell P0420.

Can somebody propose me a solution for this?

Thanks a lot!
#381 - lefteris - 06/25/2014 - 09:09
I've got a 2005 Prius with 170k miles on it. It has been setting the code p0420 every once in a while for the last two years, but now it is on all the time.

The front o2 sensor doesn't show up at all on my scan tool. Does this mean that it is bad or disconnected?

The rear o2 sensor fluctuates between .7 and .9 volts. Is that amount of fluctuation normal or is that too high?

#382 - Dan - 07/11/2014 - 10:22
cata converter
I have a 2005 saturn ion 2.2 I bought just two.weeks ago.and I.noticed it has a bad egg smell so I.replaced catalytic converter but the car still smells like eggs and now doesnt want to start...any suggestions?....also had the car checked and the mechine said it also needed sentor 1 and 2 could this be the problem?
#384 - rebecca - 07/21/2014 - 05:49
2009 Nissan Cube P0420
I replaced the upstream O2 sensor and now have the P0420. Should I replace the downstream and see if I still have an issue? Or, if I don't have a downstream code popping up, is the Cat bad?
#385 - Dale McGehee - 08/12/2014 - 20:54
p1456 p1706 on a Honda xe 1998
#386 - larry - 09/13/2014 - 14:44
Clearing P0420
I was able to clear the code. First i used cataclean and the code came back on. Next I tried Techron, Lucas oil and cleaned the air filter. It came back on. Finally; I did a synthetic oil change; put more Techron and Lucas oil and the code has stayed off and I was able to get the smog inspection done. I also drove the car on the highway for 100 miles.

#387 - Bharat - 09/16/2014 - 15:31
2003 DTS
Approaching 200K a series of issues have cropped up with this car. First an intermittent misfire after an highway trip then idling for 20 min. That led to plugs, both coil packs and valve cover gasket after finding oil in spark plug hole. Next the car was gaining speed by itself and that was traced to a leaking intake seal behind the throttle body causing a lean condition (codes)after getting the caddy dealership to do all this work the car ran great but idling in traffic set a 0420 code.
Went back and the \"best guess\" was a 2200.00 converter replacement. I went to another shop and using a Mac scan tool discovered there was no data stream coming from the downstream O2 sensor. The code has been cleared at least six times and comes back only when idling hot in traffic. I was told the computer will disregard the sensor after repeated resets when there is no data. I ordered the sensor 80.00 and will replace it to see if it comes back to the code. Co-incidentally there was a cold start-up misfire on #1 cyl caused by coolant seepage cured by using Rislone Head gasket sealer.
#388 - Caddyman - 09/24/2014 - 11:57
2003 DTS
Northstar engines as well as others are notorious for head gasket problems and the anti-freeze getting into the cat and sensors may contaminate them causing the code as well. If you have a small amount of white smoke try the gasket sealer before spending $$$ on the electronic repairs. Buy the best one available not the cheapest and only do it one time. If you have lots of white smoke the codes are the least of your problems. Good luck!
#389 - Caddyman - 09/24/2014 - 12:12
Nissan Maxima 1998

My engine light just came on. I bought this car a few months ago from a used car dealership. I have keep oil in it regularly and even go to a great place that told me that nothing was wrong with it a few weeks ago they just recommended that I use high grade oil.
#390 - Albert De Guzman - 09/26/2014 - 07:21
2003 VW Jetta P0420
Replaced thermostat $10. Had no other codes and vehicle operated fine. After I replaced pre O2 sensor, code can back right away. Researched online. Scan tool showed engine
never reached 190 degrees operating temp even
after driving on highway. New thermostat installed and reached operating temp in less than 5 min. code P0420 has not come back.
#391 - Chris - 09/29/2014 - 15:46
p042 won't disappear - camry 2004
Changed out my o2 sensors, there are 3 of them. 2 of them are Bosch and 1 denso. Check engine light p042 code came back on. Noticed that the spark plugs had oil on them so it turns out that I needed new seals. I also replaced the spark plugs. Cleared the code. Still the code goes back on. I also recently passed a smog test. So what else could it be?
#392 - tony - 10/05/2014 - 19:52
BMW 740I (1997) P0420
The Diagnostic Trouble Codes shows

Vehicle Status
Vehicle OK
# DTC 1

Readiness Indicators
Misfire - Complete
Fuel System - Complete
Comprehensive Component - Complete
Catalyst - Complete
Heated Catalyst - Not Supported
Evaporated System - Complete
Secondary Air System - Not Supported
A/C System Refrigerent - Not Supported
Oxygen Sensor - Complete
Oxygen Sensor Heater - Complete
EGR System - Not Supported
#393 - JOHN - 10/07/2014 - 06:07
is it the cat or o2 sensor
Have 98 accord 2.3 liter i took car to local fairgrounds for free emissions test. I was told emissions passed but with engine light n car will automatically fail smog. I get p0420 code. Im thinking its probably the o2 sensor since emissions passed but friend says its the cat. If it were the cat wouldnt the emissions have failed? Please help asap my smog is due now.
#394 - atina - 10/15/2014 - 14:03
po405 trouble code
Alot of premature misfires are happening on chevy S10s if not fixed in due time you will ruin the cat converter code PO405! never mind the 02sensors
Those are not the problem. !! All the unspent fuel
Just went to the cat converter and saturated it you\'ll have to buy another one period!!!!
#395 - vinny - 10/17/2014 - 20:18
DTW codes PO420
Correction the code is not PO405 it's PO420
#396 - vinny - 10/18/2014 - 09:28
engine light on only in Summer.
As soon as the weather gets over 50degrees F the engine light comes on and stays on. About November when the weather gets below 50 degrees F., I take off the groung cable from the battery , leave it off for 10 minutes and then reattach. The computer is reset, the light is off, and as long as the weather is under 50degrees, it stays off, sometimes for 6months(all winter). Why does it do this?
#397 - John Harcourt - 10/21/2014 - 13:10
To save time and money, the 1st thing I suggest you all do is buy 1 can of carb cleaner. Unplug O2 sensor wires carefully, remove the sensors and spray them very good and wipe them off. Repeat this a few times, and then clean out the connectors with the spray also. Install the sensors and plug them back up, making sure the wires are not stretched tight or against the exhaust. Plug the scanner back up inside the car and clear the codes. Drive the vehicle and if and when the light comes back on, more than likely your cat converter is done. If you need to replace it, go to ebay and order a oem bolt on replacment cat. It is much cheaper to go this route to fix the problem. If your vehicle has more than 80k, most likely it is the cat performance. Even tho you feel no performance problems and your check engine light is on, you are losing gas mileage. I hope this helps someone.
#398 - jbturbobuick2 - 10/23/2014 - 08:44
check engine light on
yes I got a code p0420 it came up to catalyst system efficiency below threshold bank mil. I like to know what that is and what I need to do to find out and the check engine light is on. my car is a ford focus 2000
#399 - jeffrey robinson - 10/23/2014 - 21:03
#400 - GARY PETERSON - 10/25/2014 - 17:52
Technician that owns a 2003 Buick LeSabre Custom
I have a 2003 Buick LeSabre custom with a 3800 V6 engine. After getting my 1st P0420 at 104K miles and the factory-original catalytic, I replaced the catalytic with a Walker 16385 direct fit (best?), Denso OEM upstream O2, Denso OEM downstream O2, MAP sensor, new PCV valve / O-rings. I replaced the MAF sensor, cleaned the throttle body. There are no intake manifold leaks (gaskets and bypass have been replaces 2X), engine uses 1 quart oil per 4K miles, runs smooth, no antifreeze loss whatsoever. I recently used Cataclean and the P0420 is back on after a month. Given that it is running very smooth with good MPG, any ideas greatly appreciated.
#401 - Paul Powers - 11/02/2014 - 16:00
i smells gas ween i turn my truck off ounder the hood
#402 - terry couey - 11/16/2014 - 04:59
Ford Taurus Codes
Hello I recently purchased a car and about a week after I got it the engine light came on. It is a 2005 Ford Taurus, so I bought it back to the guy who said I had a month warranty and said it was just a misfire and reset the code, not wanting to spend the time taking it to his mechanic, which got me angry and we had some words. A week later the light came on again not wanting to go to him I was going to bring it to autozone, but the light turned off by itself in two days. Only to come on again 2 days later, this time I had the codes read and received P0300, (random misfire), P0301 (cylinder 1 misfire), and P0420 which is what this site is about. My question is could it just be caused by the misfire and should I first inspect the plugs to make sure they are all good. Or could it of been a misfire the first time and now that he reset it this P0420 code came on because I did not deal with the misfire at that time. It is still technically under warranty but I have a feeling this guy is just another salesman who goes back on their word, I hate to say it like that no offense to any good honest salesman out there. But can this problem be fixed by just fixing the misfire or is this a separate problem that will definitely need to be addressed. Thanks for any help.
#404 - Steve A - 12/20/2014 - 10:00
Check Engine Light
My check engine light went on. I had someone diagnose it and came up with a code of P0420. What exactly does that mean? I had the NOC sensor changed about 1 year ago.
#405 - Barry Greenberg - 12/27/2014 - 16:48
Subaru owner
PO420 code is on. Replaced rear 02 sensor. Cleared code and light stayed off for abt 30 miles, then came back on. Getting ready to replace front 02 sensor. I was told to drive the car for at least 30 min to clear things out, then the light should stay off. I didn't do that after replacing the rear 02, could that be why the light came back on? Is it likely then, that replacing the front and running it for 30 min will solve the problem?
#406 - Sonia - 12/30/2014 - 14:08
2000 Nissan Altima
I went to get my smog test they told me I passed but I need to fix the P0420 sensor because that failed so how can I fix it on my car?
#407 - Ericka - 01/05/2015 - 13:42
Engine light on
Engine light on was preventing inspection done. Had 3 diagnostics done showing po420 code which means basically bad catylitic converter. Having no option to get it fixed, I took to the Cadillac dealer and find out that the catylitic converter is covered 100 percent under the Emissions Warrenty! So if your vehicle can\'t pass emissions test, less than 8 years old or less than 80,000 miles, your catylitic converter and 02 censors are covered automatically. Worth checking out...........
#408 - Lynora - 01/05/2015 - 18:20
Business Owner
Recently sold a Walker 16389 Direct Fit Converter for a 2006 Cadillac DTS 4.6L. The mechanic who installed it says the PO420 came back on 50 miles after replacing the Cat and after replacing the O2 sensors as well.I'm in the process of contacting Walker's tech support but thought I'd run this by you to give my customer some feedback. It's very unlikely that the Cat is the problem. What would you recommend under the circumstances. Your comments would be much appreciated. Thanks.
#409 - Ed Monkiewicz - 01/08/2015 - 16:04
My car is polo vw 2005 model the engine management lights comes up again even diagonustic changed convertor also again it\'s lights comes up.
#410 - Thenappan - 01/08/2015 - 17:50
2003 Chevy cavalier p0420 code. Have new cat. New o2 sensors and still get same code. Replace ecu unit every 3000 miles because car starts misfiring. Why?
#411 - mary - 01/10/2015 - 13:35
I have a Mitsb galant 4 cylinder code is saying catalyst below effiency threshold bank 1 car is running good can someone tell me if it would be the sensor
#412 - Lee - 01/31/2015 - 19:23
'04 VW Beetle-Upstream or Downstream?
How do I know if it is the upstream or downstream O2 sensor?
#413 - Heather - 02/19/2015 - 18:09
Quiero saber como Arreglar el problema de po 420 del Nisan Sentra 2007
#414 - Marlen - 02/22/2015 - 12:30
This is obviously a stupid fault that should be ignored. Is there an easy way to jump out the o2 sensors so the diagnostic system comes to the same conclusion?
#415 - bill - 02/27/2015 - 06:44
2002 Subaru Outback 4cyl
P0420 code keeps coming on, however vehicle continues to run fine as far as I can tell. Has anyone with same vehicle had similar problem and determined the fix. Thanks
#416 - Arnold - 03/10/2015 - 11:48
i have a 2002 acura and and it failed the inspection because of the code p0420 what does this mean.
#417 - niyah - 03/18/2015 - 12:18
Junk 2003 dodge grand caravan
Replaced catalyst,up and down stream O2 sensors, leak detector pump,charcoal canister,fuel pump, fuel filter, gas cap, pcv valve,spark plugs,IAC,cam shaft and crank shaft sensor,leak detector valve and I still get this stupid code.

Think it's a factory code to scare you into buying a new car.
#418 - sam - 03/22/2015 - 17:09
03 astro
My van trow a p0420 code for a few days and it went off again and a week later come on again sure I have to replace the catalytic converter or can be something also any suggestions please thank u
#419 - luna - 04/18/2015 - 18:39
Catalyst System efficiency below threshold bank 1
I got the above error code. I still have the honda 5 years powertrain warranty on it. Is this covered?
#420 - Dennis - 04/20/2015 - 11:24
code Po420
i have a BMW 2003 745 LI check engine light came up .i find a code i need to change the o2 sensor or the cat? give me an answer please
#421 - Guivard Pierre - 04/23/2015 - 20:29
P0420 code
Hi i have ford focus 2003 pj03 ywa with dtc code p0420. I need to replace lambda sensor but i dont know which one to buy from your website as there are many options available. Can you help please. Thanks.
#423 - Dhaval pandya - 05/05/2015 - 07:56
P0420 code 1998 mercury sable
I just had my catalytic converter fixed 2 years ago, cost me probably $500-$600 and my car just failed the emissions test this year. Now what. I JUST had it fixed and now its "broken" again. Now what?
#424 - Dawna Allen - 05/22/2015 - 19:57
Engine light and cruise flashing 2004 liberty
#425 - anthony wood - 05/24/2015 - 20:07
I have DTC P0420 Bank #1 twice 1# confirmed, and the other pending. what can be the problem.
#426 - William - 05/27/2015 - 19:10
po 420 code appears.what are my options.
#427 - robin - 05/31/2015 - 20:06
trying to help my elderly mother get here car registration. her test failed in order codes P0420 next P0463; only two. P0420 had come in in past and reset itself and no will no reset. car is the 2006 chevy Malibu with 2.2l 4 cyl engine. I was told by local mechanics that this engine cat converter was the problem at the engine/exhaust manifold connection sealing and it will cost a lot to fix. I would appreciate your suggestion and thanks for your website.
#428 - tom phifer - 06/09/2015 - 19:20
#429 - john - 07/01/2015 - 20:42
ayuda p420
#430 - eduardo - 07/05/2015 - 12:31
What does a ECM reflash mean?
#431 - Robert Markham - 07/05/2015 - 14:07

I have a 2003 Jeep Liberty it started making a knocking noise and the machine at autozone throws a code of it being the spark plugs if the spark plugs were bad and misfiring would it knock? TIA
#433 - Me - 07/08/2015 - 23:32
hello, Agent 2009 Ford Flex Limited models. P0420 gives you an error code. engine light is on. I've changed the o2 sensors, I checked leaks, the problem still continues. performance and rubies in my car no problem. I wipe away the lamp lights up again after 40-50 mph. What else should I do?
#434 - akif - 07/19/2015 - 19:49
P0420 and P0300
Replaced 330,000 mile 2.7L V6 in 2005 Kia sportage that had bad head gaskets, with LKQ certified 48,000 mile engine of the same specs.

Everything is brand new accept catalytic converters and 02 sensors (to cut cost of swap)

Upon completion, as soon as the car turned over I have a P0420 code, didn't want to spend the money so the car sat, I used it for some very local trips and now p0300 is present.

Is it possible the old engines blown head gaskets destroyed the catalytic converters and 02 sensors, causing my random misfire?
#435 - Glenn - 08/04/2015 - 21:01
Good knowledge website
#436 - Charlie Pak - 08/06/2015 - 23:35
Easy O2 sensor test
On1998 gmc 5.7 I swapped the 02 sensors from bank 1 and 2. If 02 is bad should read bank 2 now which it didn\'t but the code was removed. I believe my O2 sensor wire not connected correctly.
#437 - Randy gustafson - 08/11/2015 - 19:46
False code
I wanted to make mention that I recently had codes for a bad egr valve and a bad catalytic converter on my 2006 dodge ram 1500. A mechanic checked it and said both would need to be replaced for $1000. Vehicles have a separate federal warranty that is 8 years/80,000 miles catalytic converter replacement; mine is 9 years old so that was out. I replaced the egr valve ($34), cleared the codes, drove 60 miles w/o a check engine light, pulled in to an emissions location, and it passed!
What I learned is just what this article reads. Start with the part that is displaying a code which is ahead in your vehicle's emission system, clear codes, drive 50+ miles, and see if all else is fine before you replace everything else. I saved lots of money and hopefully you will to.
Hindsight, it just makes sense, that if something isn't performing as it should upstream that parts downstream will not give accurate readings.
#438 - JOAH SMITH - 08/15/2015 - 08:42
2004 volks jetta engine light on code p0240
Catalyst nelow thteshld giciency nank 1. Not sure where t0 begin or wat to change first gas cap 02 sensors before n after cat converter
#439 - sandi - 08/17/2015 - 01:16
mil light on p0240
Mil light comes on shows code p0420 but maybe a day later mil light goes out then later comes back on same p0420 code
Is this an intermittent problem or what please help drivin me nuts
#440 - Chris - 08/24/2015 - 22:51
Bad article
If you are getting this code it is because the Catalytic converter is bad. The O2 sensors throw a different code. Who ever wrote this does not have a clue
#441 - Mikee - 09/02/2015 - 20:43
whole right side not firing!
1998 Chevy /Silverado-1500 v8 5.7L that the whole right side not firing!?
i changed spark plugs& wires oil filter, fuel filter coil 2 senors above cat. what else i need to do
Update: i took it to autozone before doing any thing it had 10 code what he said mult-misfire lean one on top cat sensors no read on a other after changing all what is about it when to no fire on the only right side and so i change the sensors and no change
#442 - david - 10/17/2015 - 10:05
Catalyst System opening below toehold (Bank 1)
P0420 History

How much will cost to fixed it ?
#443 - Chowdhury - 10/30/2015 - 07:24
If catayst system not working will i brake down very fast
#444 - Martin - 10/31/2015 - 14:49
Confunsed? Get a mechanic!
This poor page has been littered with helpless comments for 6 years. Word of advice to all people... If you CANNOT understand this mans simple instructions for gods sake call a mechanic!
#445 - mech-man - 11/19/2015 - 15:56
Really Generic Advice
This webpage must've been started as a revenue stream with no regard for improved content. Since quality Oxygen sensors can cost $240 each I recommend NOT replacing them unless they are visually burned out because this code may not have anything to do with those sensors and $240 part is high high dollar. Ur, better off going to the junk yard and pulling an 02 sensor off your vehicle to swap them out. There are places like ebay to buy cheaper sensors but those may be cheaper because they break every year.
This particular code should probably be diagnosed with the scan tool to compare the two sensors and even a visual inspection inside a bolt on catalytic converter. A Converter could cost $500 and another $500 for the sensors so that's $1000 plus $200 in labor. if you want to replace stuff to save the $75 shop inspection charge. Not good economy, especially since you may actually be replacing quality working parts with Chinese junk that will soon break...and you haven't even fixed the real problem which is some dangling wire or maybe a coolant issue or even the gas cap is not on tight enough or the fuel tank is not air tight. I don't trust mechanics but that's because I'm a mechanic and I know what it really takes to do a job right and sadly $75/hr isn't enough to do the job right because they have to either cut corners or ignore blatant problems that aren't on the work order. People bring filthy bad running cars in and expect it to be fixed right for $300? No chance. It would take a whole day...maybe $1500 in labor and $400 worth of parts to resurrect a car worth only $1000 so the mechanic ignores everything but the problem the customer is complaining about, and that leads to never ending problems. But people are cheap and also ignorant of what it takes to be a mechanic.
#446 - oggy - 11/20/2015 - 19:42
car never ran any better, 4th gear 120 mph
oil change and check the plugs should be good for 1/4 mile.
#447 - bodacious - 12/02/2015 - 12:44
how to solve that error
#448 - Adnan Tariq - 12/16/2015 - 08:49
p0420 code 2003 toyota Solara 2.4l
4 years ago my 2003 Toyota Solara 2.4l had
check engine code P0420. Replaced the Catalytic Converter. Check Engine light came back on recently with P0420 code again. I have
220,000 miles on my car. Do I have to replace the cat again?
#449 - Devon - 12/28/2015 - 19:11
High fuel consumption
My camry 2003 is consuming too much fuel. The diagnosed code is P0420. What dk I do
#450 - Zaki - 01/17/2016 - 09:57
'97 Dodge B3500, 318, both 02 sensors replaced within the last month, system reset and no light. A week or so goes by, light is back on, just after a refuel. Light is still on.
Stopped at O'Reilly's, it shows the P0420 code, nothing else, no 'bank' error. It runs well, nice and smooth, although it's hard to start. The only exhaust system problem I'm aware of is a hole in the muffler, just haven't had a chance to replace it.
Could that be causing the malfunction code?
As an aside, the t'stat is underrated apparently, cooling system is only running about 175, but it's too inaccessible to worry about in a Midwest winter with no garage. More heat would be nice though....
#451 - Michael Vinson - 01/19/2016 - 15:25
I have a 2007 Honda Odyssey. Took it in last week for check engine and was told that it was the Sparkplugs. 2 days later Check Engine light is back on now it is code P0420 and Catalytic Convertor. Problem is they charged second diagnostic. Car is well over government 80,000@ 139,5K. Will be upgrading gas and running for few weeks. Any other recommendations? Being quoted $900 plus dollars.
#452 - Campbell - 01/28/2016 - 11:27
Relationship between P0420 and start up
I have a Ford Fiesta 2013,
with MIL on. Diagnostic P0420.

Is there a relationship between this error code with this two symptoms?:
1- Problems with start up.
2- Problems with change the lever in the start up too.
#453 - Mauricio - 02/10/2016 - 09:03
Both P0420 & P0430 codes jeep
Both P0420 and P0430 codes came at same time on a 2007 Jeep Wrangler, 150 K miles and an after market performance cat back system.
Does fact that both codes (left bank and right bank) came on give any hint since each side has differect cats.
#454 - Mike - 03/05/2016 - 15:33
Are there any other symptoms to look for in a catalytic converter? If it is stopped up will it blow air through the exhaust? I replaced my mass air flow sensor, spark plugs, hoses, both 02 sensors and I checked for air leaks and I do not see or hear any difference. What else could cause this p0420 code? I noticed I am getting high long fuel trim around 23% sometimes 24%. Today after a drive I sat in a yard and noticed my long fuel trim went up to 40% then finally back down to 23%. What could be causing this?
#455 - David - 03/05/2016 - 15:39
Engine Light
p0420 99 chevy astro. changed the 3 o2 sencor and catalytice converter. no more engine light. got a unversal Eastern Catalytic converter.CARB. dont buy a cheap one. thanks guys
#456 - Antonio - 03/09/2016 - 13:09
P420 on 2005 Subaru Forester
Just passed emissions test despite having a 420 code/CEL for the past year. The 420 was the only code detected, until I took it into a repair facility for diagnostics. They found an additional code related to the EGR system. Given the cost of a new converter, I decided to fix only the EGR and the front brakes. I searched for a lower cost alternative to the OEM Subaru converter ($1,200 part). Turns out I didn't need the converter. CEL came on, but I cleared it. Passed test this AM.
#457 - Jim - 03/14/2016 - 13:03
resolution code P0420
the resolution for P0420 is the computer temperature switch, if the computer is not reading at optimal temperature run that means it is giving fuel in closed-loop which is a rich condition which will continuously cause a P0 420 code
#458 - bighorn6942 - 03/20/2016 - 12:38
Oxygen sensor ?
The code comes on and off for my 2006 Dodge Charger SXT. When it\'s on the car hesitates to accelerate when I press the gas pedal. MPG also takes a severe hit and drops. Once the code goes away the car returns back to normal without any problems affecting it. Can someone please give me an idea of what I may have to deal with. I\'m going to start with the oxygen sensor replacement.
#459 - Jose Gutierrez - 04/13/2016 - 15:20
Don't ignore a smoking engine
My wife's Toyota RAV4 now has P0420 and P0430 codes.

Here's the back story. For a long time (2 years), she was puffing smoke in the morning on engine startup. This is caused by a slow leak in her valve stem seals. Oil pools up in the cylinders and burns off on startup. She finally agreed to let me take care of it. Since I have the ability to rebuild engines, I pulled her engine, hot tanked it spotless, replaced all gaskets, piston rings, timing chain...basically, it's a brand new engine at 100,000 miles. BUT, the damage to her catalytic converter was already done. After I fixed the engine, I told her it might be a matter of time for the cat. The moral of the story is, if your engine is smoking and you ignore it, you're on borrowed time.
#460 - Rick - 04/23/2016 - 12:53
1998 Honda Civic it was a rusted front pipe!!!
Ignored P0420 code all Winter because it came and went, every time it got cold. Cleared on its own in a warm week in January, then came back. Took a hard right turn and pipe broke right in front of catalytic converter. Had a new front pipe put on and code cleared immediately!
Cold air was getting sucked into catalytic converter through rust holes!
You can buy a new Header and front pipe for a 1998 Honda Civic of of eBay for around $50! I paid a mechanic $250 just for the bottom pipe! Sux!
#461 - Jesus Ali - 05/01/2016 - 19:54
ok,coil shorted out,light came on, replaced coil, turned off light, light came back on read p2195 bank 1,sensor 1 stuck lean, changed it, 100miles same code, changed both sensors, same code, switch sideswith sensors p0420, all thisonna 05 Toyota Tacoma prerunner!!!
#462 - Perry DuPre - 08/22/2016 - 12:09
po420 code in Headlight
I have a po42o code coming from my blinker. Does the fluid need replaced.
#463 - caronfirepleasepullmeout - 09/07/2016 - 09:25
I have 2001 toyota camry le the check engine light come out after I fill up the gas tank with regular gas did you think may be the problem because the car was fine till I fill up the gas tank thanks
#464 - Roberto Gonzalez - 09/13/2016 - 11:09
I have a 2004 Honda civic ex and on the scanner it's only picking up one 02 sensor. And it's pretty steady. And there's no exhaust leak. Does that automatically mean it's the converter?
#465 - Albert - 10/25/2016 - 22:25