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Q: Code P0420 ...but on 2003 Pontiac Grand Am

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Check engine light comes on. Code P0420 is the culprit, which supposedly tells me the catalytic converter is bad. I cleared the code and drove the car for 3 weeks without a problem. I had the car smogged to register it after the 3 weeks and it passed with very good results (California). Now it is 2 weeks after the smog test and the check engine light is on again and code is again P0420. Car runs ok, once in a while there is a rough idle, gas mileage seems about the same, tailpipe color is gray/white instead of brown/black (which I think means it is running lean), cruise control intermittantly drops out (may or may not be related), once in awhile when I start the car the engine shudders. P0420 error began about 3 months ago. Car has 94,000 miles on it.
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P0420 indicates that the computer detects that the catalyst efficiency is below the threshold level. That would suggest a weak or deteriorating catalytic converter but it could be a marginal condition, and when it's good and hot it works within the limits and maybe sometimes it doesn't. The tailpipe color is puzzling to me because since unleaded gas became universal many years ago, all tailpipes were shades of black, not gray, white or tan like they used to be long ago. I don't know about the engine shudder, no information in your question, curious as to how old the tune up parts are and how clean the motor oil is. It's very likely you could use a new cat, I like to go to high quality exhaust shops and get a good, expensive (but cheaper & better than original equipment) catalytic converter replacement, myself.
Here's a directory link if you want to find a GM specialist to check it out:
http://repairpal.com/directory?address=94087&car_brand_names=Pontiac
I too have the same issue with the code P0420 appearing, though less than a year ago, I had cat problems to the point where the block cracked. I put a good deal of time and money into repairing back to normal. Got a new block and converter, but now the problem seems to have returned. Is there something else besides a weak cat that could be causing this code to come back up?
I posted the original question. I too wonder if something else could be causing the P0420 code to come up. The generic answer seems to be change the converter but I don't want to end up with the same result as you did. Someone please let us know if there may be another reason for this code.
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