My dad's '02 Monte Carlo LS with the 3.4 and 92,000 miles throws a P0420 about once or twice a week. General consensus seems to be that this will happen with anything but a GM factory cat converter. I got under it and looked, low and behold, it has an aftermarket cat. We've run 3 tankfulls of gas through the car since he got it. We got 25, 26, and 27 mpg. Power is excellent, the car runs very well, there is no rattling from the cat. The question is should he worry about the code any longer (he was thinking of having the converter tested) or forget about it since it obviously shows no symptoms of any kind? Also, I read one might be able to have the code deleted completely with the right kind of equipment.

 

How did you get the DTC? Did you go to a dealer, or to Autozone or something? Because either one should have cleared the code, and if they did then it keeps popping up you need to take care of it.

You need to see if either of the O2 sensors are bad or if the cat is bad. Once you fix the problem, you should theoretically get even better mileage because this can effect performance.

 

If you don't have obd2 emissions, I wouldn't worry about it. More then likely is the after market cat causing it. 9 times out of 10 a bad o2 sensor will set an o2 sensor code. My 3 inch zzp downpie with high flow cat set the code. We have obd2 emissions here so I got the zzp 1.0 pcm which eliminated the code

 

I check the codes at home with my own scanner and reset them. The code doesn't come back for days. I've never really made note of how many miles his car goes between the code coming back.