What year is your car?
The rear 02 doesn't really do anything for performance or your engine, it basically is there to make sure you have a cat or that its working halfway decent in basic terms. Most people just buy an 02 simulator instead. 02 simulators run about $45 new, you just plug it in and tie the wire up in the engine so it doesn't get burnt by the exhaust manifolds. Then either clip the wire off the bad 02 and leave it in, or pull it out and put a plug in.
02 simulator is probably a lot cheaper than a new 02 sensor, and you dont have to worry about it again.
Now on 01+ cars (correct me if im wrong on the years), the pcm goes through a diagnostic check when it senses the rear 02 is bad, causing the engine to run rich and sometimes at idle it will float the rpms up and down. As long as the pcm is getting a signal from the rear 02, it will not do this diagnostic cycle, so the simulator is always needed, even if you have a tuner to delete the codes, it will not go away unless there is a sim.
