1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor
in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101"
, if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.