Oxygen Sensor (O2S)

The O2S is mounted in exhaust system where it monitors oxygen content of exhaust gases. Some vehicles may use 2 oxygen sensors. The O2S produces a voltage signal which is proportional to exhaust gas oxygen concentration (0-3) compared to outside oxygen (20-21). This voltage signal is low (about .1 volt) when a lean mixture is present and high (1.0 volt) when a rich mixture is present.

As ECM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a reference voltage supplied by the ECM on the O2S signal line. This is referred to as cross counts. A problem in the O2S sensor circuit should set a related trouble code.