Oxygen Sensor

CAUTION: Measure Oxygen Sensor (O2S) voltage with a Digital Volt-Ohmmeter (minimum 10-megohm impedance) only. Current drain of a conventional voltmeter could damage sensor.

The O2S is mounted in exhaust system and monitors oxygen content of exhaust gases. The oxygen content causes the 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 ECM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by ECM 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 ECM 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 ECM reads a voltage signal greater than .45 volt from the O2S, ECM begins to alter commands to fuel injector to produce a leaner mixture.