Oxygen Sensor (O2S)

O2S 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%) compared to outside oxygen (20-21%). This voltage signal is low (about .1 volt) when a lean mixture is present and high (about 1.0 volt) when a rich mixture is present. As PCM/VCM 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/VCM on the O2S signal line. This is referred to as "cross counts".

The O2S does not function properly (produce voltage) until its temperature reaches 600°F (316°C). At temperatures less than the normal operating range of the sensor, vehicle functions in "open loop" mode, and PCM/VCM does not make air/fuel adjustments based upon O2S signals, but uses TP and MAP or MAF values to determine air/fuel ratio from a table built into memory. When PCM/VCM reads a voltage signal greater than .45 volt from the O2S, PCM/VCM begins to alter commands to injector to produce a leaner mixture.

Once vehicle has entered "closed loop" mode, a fault in the O2S circuit (cooled-down sensor or open or shorted O2S circuit) will return vehicle to "open loop" mode. A problem in the O2S circuit should set a related diagnostic trouble code.

On most engines, O2S uses an internal heating element. This type of sensor is referred to as a Heated Oxygen Sensor (HO2S). Heating element allows HO2S to warm more quickly, causing fuel system to enter "closed loop" mode sooner. Heating element also prevents fuel system from re-entering "open loop" mode, which would be a normal response to prolonged idling.