Oxygen Sensor Graphs - Please help

Can anyone please help me understand my Pre-Cat and Post-Cat Oxygen sensor outputs! I have a P0420 error code. I have done everything (NEW spark plugs, ignition coil, Cataclean, servicing etc.) except put in a new-cat…but still cannot get rid of the code. Please help me diagnose the Oxygen sensor codes, so I can make a decision on what next! (see attached: Idle and ~2000 rpm)


Hello, Thanks for your reply. I have seen these a lot on the internet. Can you please look at my O2 readouts and help me understand if it is good or bad? My post-cat seems to run low voltage when idling…is that a good thing?

The downstream O2 sensor is indicating the cat bad.



I’d replace the downstream sensor first. It is responsible for about 60% of P0420 codes.

That makes so much sense (logically). I changed both the sensors about 4 years ago (1 year apart from each other)…that is a cheap and easy fix/gamble before I do anything else. Thanks :slight_smile: