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I just purchased a 04 GP GTP and I have been trying to get rid of p0420. I took it to a mechanic who looked at my cat and drove around and said he didn't think it the cat was bad and it was it was at the "beginning stage" Since then I've replace up and downstream o2's and exhaust manifold (which had a nice sized crack) I've erased the code and 30 miles later it's back. Anyone else have this problem or have a suggestion?
 

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I had my cat converter clog on me when I was getting on my car on the expressway. Immediate smell of asbestos, and restricted acceleration. I knew immediately that it was clogged. There wewre no codes.
Needless to say, ZZP has a nice way around and through that problem. Along with a long crowbar...problem solved..... and a big performance boost

You may still have a cat converter that is failing...best of luck
 

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From my perspective (professional tech/shop owner), it sounds like you have a bad cat (assuming there are no exhaust leaks in front of the cat). And maybe the tech didn't really understand how the numbers from O2#1 should compare to O2#2.

So, if there are no other codes and no exhaust leaks, get a cat or have someone with a PCM tuner remove that code from the PCM.......

Good luck, Jake
 

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I'm having the same problem with my '02 Camaro. The car runs fine, just the annoying service engine light. Erasing the code solves the problem for 150-225 miles and the car passes emissions in this period. On a long, fast highway trip the code stayed off for 2100 miles. I have headers and a large diameter exhaust so the whole system might be running a bit cooler than normal, but who knows. Output from the O2#3 sensor mirrors the output from the #1 and #2 sensors in the headers. I'm told that the #3 sensor should be reading a bit lower than the upstream ones. Any ideas?
 
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