I have a 2 1/2 in. test pipe in my 2000 ITR and soon after I put it on the check engine light came on. It baffles me how knowone else hasn't experienced this as well. I have a 4 into 1 JDM header with a 2 3/4 in. collector leading to a 2 1/2 stainless steel test pipe which connects to a Tanabe Racing Medallion cat back 2 1/4 in. system. It is really, really loud.... but it came with a silencer, and with the silencer on, it almosts sounds stock again, and is still fast as hell. 8)