So after driving on the highway for about an hour I got a code that disappeared with time, a p1131. Recently I got a new code p0420 and the p1131. I called the dealership and asked about fixing it and they suggested going with a new Cadillac converter and an O2 sensor. So my question is, since I already own a catless up pipe and down pipe, would it be better to have them installed over a new Cadillac converter. I know a shop that does work on subaru who quoted me about 400 for the up pipe and down pipe install so the cost of going aftermarket is 400 plus 650 for the AP and 100 for the 02 sensor, or is it better to just go with what the dealership recommends, or is it that the problem is something completely different. My WRX is completely stock other than an 05 engine (ej20) and turbo combo, and the car has 147000 miles on it.