Oxygen Sensor (O2S)

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

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 0.1 volt) when a lean mixture is present and high (about 1.0 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 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 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.