2001 Pontiac Firebird - Code question

2001 Firebird 3.8 litr V6. Bought new. 107k miles. Maintained by the book. Service Engine Soon Light on. No reduced performance. Local service shop I know says all sensors check ok, must be the converter. Took to a national service center who said their $8000 machine could find issue. All sensors check ok, ran heat test on converter, input, middle and output of converter heat is normal. Converter is ok. Their machine says ALL tests on the car are normal. I changed gas cap. Light still on. After the services ran test they reset the Service light and after 50 miles it comes on again.The car runs great just like new. Any ideas?

How about telling us what fault code you have?


Went to Oreilly’s and the code was P0420.

O’reilly’s said they cannot reset the codes.

Thanks for any ideas you may have.

Anthony Soule

If forwarding this e-mail please remove my name and e-mail address.

Well, that code means low efficiency for bank 1 catalytic converter

The pcm uses the downstream oxygen sensor to monitor the cat efficiency

The downstream sensor signal should be very steady, unlike the upstream sensor, which should be rapidly fluctuating, from below 200mv to above 800mv

When the downstream signal starts fluctuating at a rate very close to the upstream sensor, that code will set

Somebody knowledgeable needs to diagnose this

Sometimes it’s the sensor(s) at fault

But very often, it’s the cat itself

A dyno run using the 5 gas analyzer would also tell a lot, again assuming the guy knows how to interpret the results

That “heat test” merely indicates that SOMETHING is going on inside the cat

it doesn’t prove the cat is effective enough to stave off that P0420 code

By the way, replacing the gas cap has nothing to do with the code . . . at least it was cheap

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Thanks very much for the response. Sure a lot more info than I have gotten from the “service centers” I have talked to. I’ve been searching the web for a place that I think could really understand how to analyze the problem.

I guess with 107k miles it probably is the cat. I just hate to think some guy is just cherry picking by changing it without really knowing. I had a Midas guy wouldn’t even change it out. Says he just won’t do it as he has done it and the car keeps coming back in to reset the code. Sounds like he really didn’t know if the cat was bad ???

The Service Center that said everything checked OK wanted me to bring the car back in and they would run the test again for another $103. I cant imaging it would turn our any different. This is the place where the manager wants to buy the Firebird from me. It is a cherry.

Guess I could take it to the GMC/Pontiac Dealer where I bought it. Lord help my pocketbook.

Last question: What is your opinion on on cat replacement with OEM or after market Cat?

Thanks for all the information. Really appreciate it. You obviously you know your business


I think you should bring your car to a reputable independent shop, not Midas, not even the dealership

Midas is known for installing mediocre exhaust parts, not diagnosis.

It proves a few things.

His diagnostic skills aren’t that great

he also doesn’t even know how to verify the repair

The dealership is probably going to have a young guy working on the car, who’s short on diagnostic experience, but long on guesses which will cost you plenty of money

A thought . . . go to a shop which specializes in smog inspections and repairing vehicles which have failed smog. Realistically speaking, smog technicians have a good understanding of how emissions systems work, what causes failures, how to interpret things, how to diagnose and how to verify a repair

It’s quite likely no oem cat is available for your car, due to its age. Aftermarket cats are fine. A direct-fit part would be the best, but most expensive, option. But if somebody can do a good job welding in a cat, that’s fine, too. There are a lot of manufacturers out there, some are better than others. If the shop determines the cat is in fact the problem, tell them to install a good quality part, because the car is a keeper. Make it clear you’re not looking for a cheap repair, for the purpose of flipping it or passing your problems onto somebody else. But be nice about it. No guy with any pride wants to the customer to suggest he’s willing to perform substandard

What sensors would those be?

If you did bring your car to one of those shops I mentioned, the one that does smogs and diagnoses and repairs vehicles which have failed smog . . . they should be able to do a pre-test on the dyno, which will give them a printout of the 5-gas analyzer. The fee would be approximately the same as for an actual smog test, but no results would be submitted to the dmv

If you go that route, please post the results

Is that because the downstream sensor’s signal mirrors the upstream sensor’s signal?

There are two choices in aftermarket catalytic converters.

You can either get a cat with a 25,000 mile warranty, or a cat with a 50,000 mile warranty.

Price dictates the warranty.


A shop with the proper equipment can easily tell you if the problem that caused the p0420 is the cat or the O2 sensors. I expect it is the cat in this case. Not rocket science, they just compare the voltage vs time signals for the proper relationship between the front sensor and the rear sensor. No need for guess-work. If a shop says they’ve replaced a cat and it didn’t fix the problem, they may have not had the proper test equipment needed to assess the cat function in the first place. A dealership would have the equipment, or most major inde shops who specialize on American cars would it. Just ask them if they have the manufacture’s scan tool for your car.

You don’t need a Tech2 to diagnose this particular problem

A professional level scan tool . . . there are a few different choices . . . will give you all the data you need

Of course, the guy using it has to be smarter than the machine :smirk_cat: