Oxygen Sensor: Description and Operation

The exhaust oxygen sensor is located in the exhaust system and monitors oxygen content in the exhaust gas stream. Varying concentration of oxygen in the exhaust gas causes the O2 sensor to produce a varying voltage output. By monitoring the O2 sensor output voltage, the ECM can determine the amount of oxygen in the exhaust gas and adjust the air/fuel mixture accordingly.

Rich air/fuel ratio = low oxygen content and high signal output (0.45-1.0 volt)
Lean air/fuel ratio = high oxygen content and low signal output (below 0.45 volt).