A code P0420 may mean that one or more of the following has happened:
Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The temperature sensor is not working properly
Damaged or leaking / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Some suggested steps for troubleshooting a P0420 code include:
Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter
One thing to note is that many offer a longer warranty on emissions-related parts. So if you have a newer but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.
OE cat converters have a mandatory 8 year 80,000 mile warranty from the factory. 99.9% of the time a catalyst efficiency code is a bad cat. You need to be careful some cars have more than one cat. The one the light is referring to has to have a o2 sensor before it and after it.
ASE Master Tech, L1 Advance Engine Performance certified
I will not answer repair questions by PM, if you would like my input you can PM me and ask me to reply to your public thread.
I have also seen this code be set with an aftermarket O2 sensor POST cat.
Had a customer with a dead O2 sensor, replaced it with a bosch sensor, and they came back the next day with this code. The O2 readings, instead of being dead, it would switch too fast. The post cat O2's are designed to be "lazy". Switched it out with a Delco and never saw it back
If you have a question, ask it in the forums. I will no longer be answering questions sent to my email.