Well he said hes using a test pipe...that means that the 2nd O2 sensor(I'm guessing s2k's have them). Is going to throw a code because the lack of the catalytic converter and cause your a/f to lean out. You'll need an O2 sensor simulator they aren't really expensive. It basically tricks your 2nd O2 sensor into thinking that your a/f is fine. I'm not to familiar with s2k's but that is the basic stuff