Does anyone know how to check if the O2 sensor is working?

 

Go to this site http://home.rmci.net/panther/ecu_codes.htm and make for less than 5 bucks a ECU code downloader. Or, if you have a digital meter, go to pin 2g on the ECU, and with long leads on your meter put the meter on the dash and drive around and watch the meter. It should rapidly vary above and below STOIC (about 40 millivolts). If you make the downloader one of the two led's will go on and off at a given rate. At idle the oxygen sensor is inactive so make sure your driving or reving the engine.