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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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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.