A code P0420 may mean that one or more of the following has happened:
A damaged or failed oxygen / O2 sensor
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
Failed or underperforming catalytic converter (likely)
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings Leaking fuel injector or high fuel pressure Cylinder misfire Oil contamination
Some suggested steps for troubleshooting and fixing 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 steadier).
Inspect the downstream heated oxygen sensor, replace if necessary
Replace the catalytic converter
Generally speaking, you can look at the temperature of the exhaust just before and just after the converter using an infrared temperature gun.
The output should be about 100 degrees Fahrenheit hotter when the engine is fully warmed up.
Using a OBD-II scanner, determine if the long term fuel trims are too rich or lean