A code P0420
may mean that one or more of the following
- Leaded fuel was used where unleaded was called for
- A damaged or failed oxygen sensor (HO2S)
- Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
- Retarded spark timing
- The oxygen sensors in front and behind the converter are reporting too similar
- Leaking fuel injector or high fuel pressure
- Cylinder misfire
- Oil contamination
Some suggested steps for troubleshooting a P0420
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair
- 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 (HO2S), replace if necessary
- Replace the catalytic converter
Overall probably the biggest mistake vehicles owners make when they
have a P0420 code is to simply replace an oxygen sensor (H02S). It is
important to do proper diagnosis so you\'re not wasting money replacing
We strongly recommend that if you need to replace the catalytic
converter that you replace it with an OEM unit. Second choice would be a
high-quality replacement part. There are many stories in our forums
where folks replaced the cat with a cheaper aftermarket one only to have
the code return shortly thereafter.
One thing to note is that many vehicle manufacturers offer a longer warranty
on emissions-related parts. So if you have a newer car 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.