Alright guys, obd II cars have two o2 sensors. Upstream and Downstream. The downstream, or rear o2 sensor, is basically there to make sure the cat is working. He will not run rich, and not have bad gas mileage because of it. He will throw a CEL though. There is probably something you can wire in for the rear o2 sensor to make the ecu think its there and working, but I have obd I, so I dont have to worry about it