Trouble Code - P0420 Catalyst System Efficiency Below Threshold (Bank 1)

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Welcome to P0420.com, 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). 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.

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 syptoms 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 oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.

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!

User Comments

07 civic p0420/p0420pd

I recently installe 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 - 14:46

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 - 08:07

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 - 09:27

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 - 18:38

p0420

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.
DO YOU THINK I AM CORRECT ?

#5 - neil - 06/14/2009 - 06:54

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 - 12:18

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 - 11:58

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 - 16:42

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 - 00:15

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 - 12:58

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 - 18:54

P0420

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 - 12:55

p0420

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 - 14:19

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 - 18:54

2001 olds aurora

Will removing the resonator from my 2001 olds aurora turn my check engine light on.

#15 - edgar - 10/07/2009 - 22:28

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 - 09:38

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 - 06:56

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 - 10:59

Technician

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"

Raul

#19 - RAUL MURILLO - 11/03/2009 - 09:39

Frustrated

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 - 07:00

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 - 13:13

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 - 18:16

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 - 12:14

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 - 16:39

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 - 10:37

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 - 15:54

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 - 15:00

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 - 18:12

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 - 19:06

po420

I have 06 ridgeline 120,000 and got this code, what should I do?

#30 - Rich Murphy - 01/31/2010 - 17:51


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