2002 Pontiac Grand Prix

WFI

MEMBER
2002 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
102,500 MILES

Last year my wife's 02 Pontiac GP failed an emission test due to code P0420 which means " catalyst below threshold bank 1. We replaced the cat and O2 sensor ( maremont ) and it passed. Two months later the service engine light came back on and the same code reappears. No one can figure why a new cat would cause that and I've heard maremont has a good rep. It's waranteed but I hate to think of replacing it again and I suspect the same problem will surface again. Any suggestions would be appreciated.

Wednesday, December 16th, 2009 AT 4:47 AM

2 Replies

MMPRINCE4000

MECHANIC
8,548 POSTS

The O2 and CAT could be the result of another problem. For eample, any coolant in the exhaust or an excessivley rich mixture will cause O2/CAT failure.

First remove the CAT and look for melted material, cracks or chunks of material inside CAT.
Melted material would indicate a rich mixture, cracks, chunks would indicate an impact event, possibly with road debris.

If OK, then have dealer or trusted garage hook PCM to scanner to measure Air/Fuel ratio and O2 sensor readings.

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Wednesday, December 16th, 2009 AT 6:03 AM

WFI

MEMBER
2 POSTS

Thanks for the reply. The old cat was in perfect condition ( no melted material, cracks, chunks ) I've been under the car and the new cat has no marks, scratches, dents. Also the rear O2 sensor was replaced with the cat. I've been told the only way to solve the problem is with a GM cat. However, that came from a pontiac dealer so it's taken with a grain of salt. Thanks

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Thursday, December 17th, 2009 AT 3:26 AM