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Re: P0420 misinformation
Welcome to AF and the forum.
It would help to post the year and model Grand Prix along with the current mileage.
In most cases the P0420 DTC is a bad CAT converter. Oxygen storage capability is less than desireable. However, it doesn't hurt to check out the post CAT 02 sensor but if it is capable of generating the specific DTC then it is working properly.
If you have a 2001-2002 model it may qualify for a free converter under the extended emissions/converter warranty which was revised to 10 years or 120K miles, whichever comes first.
Also, you have to be careful with certain aftermarket or universal CAT converters since they may still give off a P0420.
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