P0420 CEL again...


Original poster
Oct 18, 2012
So the story is that I got the code first a while ago, changed o2 sensors and it went away for a few months then the code came back and I got a good deal on a cat setup so I swapped it (catco brand and it was new), the code went away for about 6 months but now it is back again, any ideas what is triggering it now? I've also done a check with my code reader that shows live data and it's saying that the downstream o2 sensor is bouncing around like the upstream one is, can a sensor go bad in only maybe 10,000 miles? Also as a side note the whole car seems to be "chugging" more then normal at idle, but drives great.


Lifetime VIP Donor
Nov 19, 2011
Portland, OR
I had that exact same symptom. If the downstream sensor is bouncing at the same rate as the upstream one, then the cat is either crapped out or (in my case) missing. I banged my cat housing on many rocks over the years, and the ceramic inside broke up into little pieces and departed the housing into the muffler. I couldn't tell the increased rattling from my normal collection of noises.


Dec 12, 2011
The downstream O2 sensor won't settle down until the cat is good and hot. Sometimes takes a short drive or an extensive warm up period. I would perform a leakdown test on your injectors just to rule it out or determine that you may have a leaker. The fact you keep going through sensors and cats indicates a rich condition or maybe even a faulty ECM but unlikely.

There are a few simple things you could do just to cover some bases, and that's to pull the MAP, IAT, CPAS, cam sensor, ECT, and TB harness and use some electronics cleaner to ensure good contact and minimal resistance. Then make sure your ground connections are clean, both sides..use sand paper. You can go as far as measure resistance through the sensors but I'm not sure what they should read and if that's even protocol but prob not an issue.

Then I would focus on fuel trims and see what the ECM is trying to do. Always pulling a lot of fuel from the injectors or not, others will need to chime in on that. But don't go chasing a lot of ideas until you get the harnesses cleaned up. May not be a prob but can sure drive you crazy if one has high resistance so eliminate those variables now.

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