The only reason the check engine light comes on is because the computer detects no change in the readings of the upstream (green connectors) and the downstream (blue connectors) oxygen sensors. This is all "after the fact" . . . meaning that this condition alone does NOT affect performance. You'll have a code p 0420 and p0430 in memory for catalyst below effeciency on bank one and two, respectively. The downstream O2 sensors do NOT affect performance. They are for emissions detection only. DarkfireGT stated that he had a code for the front (upstream) O2 sensor. The front O2 sensor is critical for mixture control and will cause poor performance and fuel economy. I had an o/r x pipe with no tune for about a month and I can tell you two things. First, it sounds like crap. Second, it doesn't run a bit different. I have it for sale now because I put on a Bassani catted x pipe and it sounds great. My suggestion is to stick with stock cat h-pipe and change the muff's or stick with stock muff's and change to a catted xpipe. You are not gonna get anY performance gains either way, only a change in sound. I noticed HUGE performance gains with a C&L cai and SCT2 tuner btw