my 99 hatch has no cat. Running a y7/ y8 mini-me w/ header, obd2b p2p ecu. Im in eastern canada, so there is no emisions testing. I got a 2nd o2 sensor cel but fixed it by taking a peice of 3/4" pipe, 3" long threaded to fit the sensor. I mounted this on my firewall and havent had any cel since. The car does smell a lot more from the rear(exhust). Noise level wasnt much more than previous.