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RussMasterTech, ASE Certified Technician
Category: Car
Satisfied Customers: 791
Experience:  19 years of GM Dealership experience. ASE Master with L1 and GM Master Certified.
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2001 pontiac Grand Prix 3.1 V6 - i get the diagnostic

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2001 pontiac Grand Prix 3.1 V6 - i get the diagnostic code P0420 - the catalytic converter was replaced 6 months ago and my check engine light is still on. Any suggestions what my next step should be?

Hey there Customer.

If you had an aftermarket converter installed, then the converter could still be the problem. I have had nothing but bad luck with aftermarket converters and the code p0420. I just had one at our shop where the customer was coming in to get there converter replaced under the GM special policy for the converters plugging up, but they already had replaced it with an aftermarket converter and it was causing th p0420 to set. They couldn't get the GM converter because they had already replaced it.

The trouble code chart is pretty simple on that code. It basically says, if the p0420 is set, then replace the converter. One thing that will cause premature failure of a new converter is a misfiring engine. Before you have the converter replaced again, make sure the engine is running well. Have the ignition components inspected, if the plugs are original, perform a tuneup and maybe even have the injectors cleaned.

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Customer: replied 9 years ago.
Hello, thank you for your quick response. I need a little more clarification. The catalytic converter was replaced because the vehicle was running horribly. There was a significant loss of power. Also, when driven, the car would not shift gears and it would run at high RPM's. The first step I took was to tune up the vehicle. I replaced the plugs, wires as well as the air filter. I also cleaned the fuel injectors and throttle body. After all that I realized there was no difference in the cars performance. So I replaced the catalytic coverter. That solved the performance problem however the check engine light continued to stay on. I used a hand held code reader to clear the diagnostic code. I ran it for a whole day but the check engine light never came on. But when I hooked up the code reader again to see if all the systems had run their checks the same code appeared. After I unplugged the code reader and restarted the car the check engine light DID turn back on. Any additional information would be greatly appreciated.

Thank you,

If the new converter was functioning correctly, the code should have run and passed and the light should have gone out by now. I really think if you put an aftermarket converter on the car, that it may not be efficient enough to pass the OBD2 GM threshholds. One thing you may want to consider is getting reimbursed from GM for the cost of your converter. Like I stated earlier, they have a special policy on the converters, and they would have replaced it at no cost to you if you would have taken it to a dealer. You should can a Pontiac dealer and see if you can get reimbursed.

I know this is frustrating, but sometimes aftermarket parts just aren't the answer.


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