Toyota Repair: From your experience, what has been the most common cause for the P0420?, p0420, oxygen sensor

Question
Is it the Catalytic converter or post converter oxygen sensor?

My code reader gives "Live Data" and the pre-converter sensor’s voltage varies from .115V to .935V at idle.  I have read that this is correct when the ECM is running its 5-second DTC P0420 diagnostics at idle test.  During this test the ECM is setting the mixture rich and then lean.

The post-converter sensor voltage reads between .700V and .935V but sometimes drops below .200V.   The reader unfortunately doesn’t display both sensor voltages on the same screen so I can tell when the ECM 5-second test is running.  There are also percentages displayed but no where can I get definitions of these or of what.

I will probably replace the post catalytic sensor first unless you have some helpful suggestions for me to look for.

Enjoy your answers and appreciate you staying with the program as long as you have.

Thanks - Jack

Answer
In most cases the code is set because the cat converter is inefficient, the rear sensor should not fluctuate very much, it's voltage should be steady around 1volt with very little deviation or fluctuation, if it does it means the converter is not working efficiently, because it hasn't cleaned the emissions the rear sensor reacts to this by changing it's voltage almost in step with the number one sensor. Voltage tests are not the most accurate way to check either sensor, they should be checked with either an oscilloscope or a scantool which can display the waveforms of both sensors.