Originally Posted by PR1AWRet

Unless your cats are plugged (literally) up they offer very little resistance. If your cats are unplugged (electrically) your system (OBD2) will stay in open loop mode and uses the preset maps in the computer. When these maps are used, it throws more fuel into the mix to keep it running smoothy until it gets warmed up and other parameters are met. Like a choke on carburetor, keeps it running rich until warmed up and uses a lot of gas. If ran this way (OBD2) for extended periods it will clog the cats reducing power. Haven't researched, if you can cut the aftercat O2 sensor out of the computer parameters then running without cats is no problem and will go into closed loop mode. It'll just pollute a bit more.