My check engine light came on 6 months ago and the code said I needed to replace one of the O2 sensors. I used a bottle of engine/fuel injector cleaner and it cleaned the deposits off of the O2 sensor so I didn't have to replace it. Been running fine ever since (12k miles).
BTW the sensors are supposed to last 100K miles and sometimes the error can be caused by the heater not working correctly. The heater is supposed to allow the sensor to work right after starting the engine before the engine has had a chance to heat up. If you have noticed a decline in performance then you probably don't need to replace it as it regulates the oxygen mixture for proper fuel combustion.