Oxygen Sensor

The 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 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".

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 does not make air/fuel adjustments based upon O2S signals, but uses TP and MAP or MAF sensor values to determine air/fuel ratio from a table built into memory. When PCM reads a voltage signal greater than .45 volt from the O2S, PCM begins to alter commands to fuel 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 DTC.

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.