I have the Bassani X-Pipe W/o Cats and MAC Cat Back and had the same prob as you. After I bought a chip, the check engine light shut off. But when it's cold outside, below 40 my check engine light comes back on and the car still runs great. When it's above 40 the light is off and still runs great. Here is a link to get some eliminators
http://www.mileliminators.com/ I still do not know why the light comes on when it's cold, but as long as she runs good I'm alright with it. Love my bassani!!!