P0420 Code

FMS has good info on p0420.
FSM can be downloaded here: Copy/paste:

The ECM observes the waveform of the heated oxygen sensor located behind the catalyst to determine whether the catalyst is performance has deteriorated. If the catalyst is functioning normally, the waveform of the heated oxygen sensor located behind the catalyst switches back and forth between rich and lean much more slowly. When the waveform of the heated oxygen sensor located behind the catalyst alternates flatteringly between rich and lean, it indicates that catalyst performance has deteriorated.

After engine and catalyst are warmed up, and while vehicle is driven within set value and engine speed range, waveform of heated oxygen sensor (bank 1 sensor 2) alternates flatteringly between rich and lean (2 trip detection logic)

• Gas leakage in exhaust system
• A/F sensor (bank 1 sensor 1)
• Heated oxygen sensor (bank 1 sensor 2)
• Three–way catalytic converter