you need mil eliminators (malfunction indicator light). these hook into the rear o2 sensors and fool the factory sensors. The front ones will still be fine. I think it is the best mod for these cars. almost 23 hp. don't buy mil lights used because they are prone to go bad sometimes. ebay around 60 bucks for a set. If you dont like the sound throw a set of used flowmaster 40's. gettin rid of the 4 cats will help wake this pup up.