I just got my exhaust put in today, 2.5'' Carsound cat matted to 2.5'' mandrel bent piping to my n1. This is the first time my check engine light has come on.

Im thinking its the o2 sensor, because the cat came with an airtube (not sure what it was for), the guy at the muffler shop told me I didnt need so he took it off and put in a bunge hole to relocate the o2 sensor.