Heated Oxygen Sensor

CAUTION: Measure Oxygen Sensor (O2S) voltage with a digital volt-ohmmeter (minimum 10-megohm impedance) only. Current drain of a conventional voltmeter could damage sensor.

The Heated Oxygen Sensor (HO2S) is mounted in exhaust system and monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2S to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3 percent) compared to outside oxygen (20-21 percent). This voltage signal is low (about 0.1 volt) when a lean mixture is present and high (about one volt) when a rich mixture is present. As PCM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by PCM on the O2S signal line. This is referred to as "cross counts".

Once engine has entered closed loop mode, a fault in the HO2S circuit (cooled-down sensor or open or shorted HO2S circuit) will return engine to open loop mode. A problem in the HO2S circuit should set a related DTC.