I just got new piping on my exhaust from the cat back- not for performance, the stock one was rusting. Anyway, after driving about 30 miles my check engine light went on. I took it to Auto Zone for a check and it came up as Troublecode P0420. They say it is the oxygen sensor but I am not sold on replacing the O2 sensor since someone else did and the light came back. Someone else told me to unplug my battery to reset the system. I did and it worked- for 60 miles and then the check engine came back on. I searched on the forums and have not been able to find a definitive solution to my problem. I have a Magnaflow hi-flow cat, but have had it on for a long time before this happened. When I took it back to the place that did the piping he tried saying it was the cat, pointing out how dark it was (even though that is its natural color). Any help is appreciated.

P0420: "The PCM has determined that the catalyst system efficiency for back 1 is below threshold for the current engine operating conditions. (Bank 1 identifies the location of cylinder #1, while Bank 2 identifies the cylinders on the opposite bank)"

 

remove the heat shield form your header. I have a su****ion it is cracked

 

thats funny i got a 97 dx and the engine light comes on and goes off as it pleases. i went to get itt checked out they said the same thing...the oxygen sensor....50$ later and 2 months later im back in the same boat.....on and off....the rain seems to make it more prone to comin on but w.e lol i kno this helps u in no way i jus thought it was coincidental.......

 

Last week my CEL came on. The only thing different about the cars normal routine is that I started parking it in the driveway vice in the garage two days prior to the CEL coming on.

Advanced Auto Parts pulled code : P0420
I downloaded the service manual for the vehicle via this website. Page 11-253 had me do the ECM/PCM Reset Procedure -pull 7.5 fuse that's inside the under the hood fusebox for about 15 sec, then reinstall it. Manual stated that if after two minute test at 40-55mph doesn't make the CEL code come back then it was just an intermittent failure and the system is okay.

I've driven over 100 miles since then and still now CEL code. I hope this helps someone in the future.

 

My car was pulling that code a few months back. First I replaced the O2 sensor, CEL still on after that, the catalytic converter went bad, blew chunks into my muffler. I just replaced the cat and that eliminated the CEL, just replaced the rest of the exhaust because there was cat converter pieces in the rest of the exhaust making it rattle like crazy.