well, if you remove the cats, that means you remove the o2 sensors, which will trigger your check engine light, however some companies like AA sell 02 simulators where its like a plug that you plug in the 02 sensors and it will simulate them and turn off your check engine light, that costs 150 dollars, why bother with the OBDII, and besides, the modifications that you can do on OBDI and not on OBDII is not limited to straight pipes