P0420 misinformation

Yeah, my first post! Anyway.... I know this code means that the catalyst is below threshold. It should be the cat converter, but I've also heard it could be one of the O2 sensors(post-cat) or maybe just needs a tune-up. I can get a universal cat installed for about $150. I can replace the O2 sensor for $50, or a tune-up for somewhere over $100. I don't want any of these to be in vain or a waste if it's not needed. There are no driveability problems that I can see or feel. My wife (it's her car) says it smells "hot" after driving around for a while. I don't really understand that, so I drove it yesterday with the air on high. The temp gauge didn't show high, and only a couple of times did I think I might smell *heat*. I flushed the radiator a couple of months ago and the fluid still looks clean and full. The oil mark on the dipstick was actually a little above full before I cranked it, so maybe that is what we are smelling(the burnoff)??? I was thinking about taking it to a shop and having them check the sensors and cat, but I know they are liable to tell me it all needs replacing. Money is tight, so I don't want to waste money for them to check it either. Any thoughts or suggestions?

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.

oops I meant to do that! It's a 1999 Grand Prix with a little over 181,000 miles.

Thanks for the updated info but what engine or model? Yeah based on your year and mileage we can rule out the free CAT converter.

My suggestion is go with the CAT converter.

It's a GT with a V6. No supercharger. I was figuring it's probably due replacement, but I'm pretty sure almost everything is due repl. because of the high mileage.

So.... I can get a magnaflow direct bolt in for about $175 and do it myself, or a universal for about $60 and have it welded in for $75. Those are my two cheapest routes.

What would be your suggestion? I don't want to get the check engine light again because I put a cheap one on. Is there a cheap one that works without getting the light?

I would get a magnaflow weld in, and take it to a shop. Problem with "bolt ins" is that higher milage vehicles tend to have warped exhaust flanges. So when you unbolt the old one, and bolt in the new one, you get a leak due to a straight edge bolting to a warped one. I hope that makes sence.