I'm assuming you have an OBD2 car if you have a secondary O2, right? I don't remember but I think you'll throw a CEL if you don't have a functioning/useable secondary O2 sensor on it.. I know on Focuseseses you'll throw a CEL after a little while if you have the O2 sensors too close or not coupled on either side of a cat (i.e. you have a test pipe) and I think most OBD2 cars are the same..