I believe there is a cheap fix on the cats 02 sensor which can make it not throw the CEL when u attach it to a test pipe. I think it basically moves the sensor further away from the main exhaust flow to keep the cpu thinking everything is ok. Not sure about that but you can definetly use a test pipe and not throw a CEL with something like that. Plus you can get a test pipe made at a muffler shop for cheap whereas a new cat would be in the hundreds of dollars range. Oh and you should put a 100 shot of n20 on there for good measure. I recommend spray to everybody