The oxygen sensor in the exhaust system reacts with the oxygen in the exhaust to produce a voltage output that varies depending on the oxygen content. This feedback signal is used by the PCM to determine the needed fuel injector pulse width and control the air/fuel ratio. The ideal ratio for emissions, fuel economy, and engine performance is 14.7 parts air to 1 part fuel. The post-converter oxygen sensor on this car is used for the catalyst monitor system and produces a slower fluctuating voltage signal. The PCM can detect oxygen sensor problems and set diagnostic trouble codes. To test the sensor, backprobe the signal wire terminal of the oxygen sensor connector and monitor the voltage. While pre-converter should switch actively between 0.1 and 0.9 volts, post-converter should also range from 0.1 to 0.9 volts but not switch actively . Check battery voltage supply and ground circuits to heater of oxygen sensor, check resistance of heater . In case an open circuit or excessive resistance is indicated then replace a faulty Oxygen Sensor . When replacing a sensor ensure that exhaust system has cooled properly follow proper handling procedures . Raise vehicle, disconnect electrical connector, unscrewed using a suitable wrench or socket, apply anti-seize compound at threads and tighten securely . Lower vehicle and reconnect electrical connector.