Originally Posted by Mike_Mac
P0420 is for a failed/failing cat or bad rear o2
the rear o2 should be very close to a straight line (if i remember correctly)
Yeah, that's correct. I don't really understand some of the markings on the graph but those should both be between 0-1V. The front should fluctuate fairly quickly between 0-1V and average about .45 to .5V. The rear should be a basically straight line, it's mostly there to show the PCM that the front is maintaining the correct average.
The fact that you're getting a signal means the wiring is okay, I'd just replace the rear sensor, clear any codes, and retest it however you did that graph. How did you do that graph anyway?