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!
Other P0420 reading
Here are a few links to some other sites with great P0420 articles:
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 ?
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
I am in NC and believe the vehicle does not have California Emissions. Is there anyway to diagnose thie/DIY?
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
gas in and within half a tank engine light
went out and hasn't returned try it
it might save you money and headaches
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?
I changed the pcv valve out and it fixed it.
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!!
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.
light stays on no leaks in cat or exhaust it looks good. Any advise?
Thanks Ed
See if this helps.
Good luck,
Ed
At the same time I had a PO420 code show up and my mechanic was ready to replace the CC. I have cleared the light and will see…
Is it possible the cracked air intake caused the inefficiency of the CC and what are the chances of my having damaged the CC as my mechanic is suggesting?
Thank you very much.