Oxygen Sensor

CAUTION: DO NOT attempt to measure oxygen sensor output voltage using a conventional voltmeter. Current drain of voltmeter could damage sensor. Oxygen sensor voltage signal can be measured using a 10-megohm (minimum input impedance) digital voltmeter.

The Oxygen Sensor (O2S) is mounted in the exhaust system where it monitors oxygen content of exhaust gases. Four oxygen sensors are used on some models. The oxygen content causes the Zirconia/Platinum-tipped oxygen sensor 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 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 oxygen sensor signal line. This is referred to as "cross counts".

The oxygen sensor will not function properly (produce voltage) until its temperature reaches about 600°F (316°C). On some models, oxygen sensor is equipped with a sensor heating element. This type of sensor is referred to as Heated Oxygen Sensor (HO2S). This allows the sensor to reach operating temperature sooner and prevents fuel system from re-entering "open loop" mode due to a cooled sensor (which is a normal occurrence during prolonged idle).

Heated oxygen sensors are mounted before and after Three-Way Catalyst (TWC). PCM monitors voltage produced by heated oxygen sensors and compares both values to determine catalyst efficiency. Rear mounted HO2S is normal when activity appears lazy or inactive, indicating TWC is functioning properly.

At temperatures less than the normal operating range of the sensor, engine will function in "open loop" mode and PCM will not make air/fuel adjustments based upon oxygen sensor signals but will use TP sensor 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 oxygen sensor, PCM will begin to alter commands to injector to produce either a leaner or richer mixture.

Once the engine has entered "closed loop", a cooled-down sensor or a fault in the oxygen sensor circuit (open or shorted circuit) is the only condition which can return it to "open loop". Failure in oxygen sensor circuit should set a related diagnostic trouble code.