Would this be the ever so popular P0420?
Causes
A code P0420 may mean that one or more of the following has happened:
Possible Solutions
- Leaded fuel was used where unleaded was called for
- An oxygen sensor is not reading (functioning) properly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / 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 error 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