I have a 2001 Grand prix gt 99,000 miles. I'm going by the mpg on the dash info center. I have gotten to around 17mpg in the summer (po420 code was present then also)..but now I get around 11-12 mpg. I know its cold outside now but still.
I'm looking to order a cat and down pipe from zzp..not sure if its 2 or 3 inch though.
Also will be doing a tune up asap...Not sure how much that code can affect gas mileage but I have to try something.