you could use a resistor. all you would have to do is find out the voltage a "correct" rear O2 would read and put a resistor in the wiring for the rear o2. Basically at that point you would just fool the ECU into thinking the cats are doing the job they have to, and the CEL wont come on... Im sure there is an easy way to use a resistor of some sort, and find the voltage range a rear o2 needs to be at to prevent CEL's