# P0420 code - after replacing cat and O2 sensors



## Metil (Nov 14, 2012)

Hi everyone,
Got a P0420 check engine light on my 2001 Nissan Altima GXE. Went to a mechanic and they replaced the cat, but ~1yr later the code came back. I replaced O2 sensors, but the code came back after some time. 
Now, the most weird part is sometimes the light turns off by itself - and it always happens in cold weather. As soon as starts warming up - the light goes back on.
I don't see any performance problems, or gas mileage issues. I would not even both fixing it if I did not have to pass smog to register the car.
Any ideas on how to troubleshoot it and what might be the causes?


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## ProDriver77 (Feb 18, 2013)

Did you replace it with an OEM cat or a Universal? Some universals don't play nice with the computer.

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## rogoman (Dec 16, 2004)

Cheap universal CATs don't last long; most of them have a 5 year warranty. You can simply use the warranty and get another one.

Here are some other possible causes for a P0420 DTC:
-  Intake air leaks
-  Injector leaks or bad injector
-  Spark plugs
-  Improper ignition timing


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