1997 Buick Skylark p0420

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 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

Possible SolutionsSome 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

A P-0420 code is emissions related. 99% of the time it is telling us that the rear oxygen sensor does not see that the converter is working. The first Oxygen sensor tells us that the air filter mixture needs adjusting, so it sends references to the computer for more or less air/fuel. The rear oxygen sensor tells us that it smelled the gasses from the converter and if the readings are too high, then you have "catalyst inefficiency" therefore telling us the "cat is dead" So, if you have a live data scanner and see that the rear oxygen sensor voltage is fluctuating as much as the first oxygen sensor, then you need a catalytic converter.