Is there anything i can do to extinguish a CEL for the rear O2 sensor after swapping in a test pipe? Has cat efficiency code. I thought i saw on the Net of O2 sensor "foolers" that tell the ECU a cat is there somehow. Does it affect fuel trims by not having a cat? Or will I just have to live with the CEL?

 

You could try a spacer that raises the sensor up out of the main flow so it just gets a little. I don't think it is guarenteed to work though. It shouldn't change your mileage it is primarily there to make sure the cat is working. Another option would be to get a conversion harness and obd1 ecu unless you have echeck.