Oxygen (O2) Sensor

O2 sensor, mounted in exhaust system, monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2 sensor 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 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 O2 sensor signal line. This is referred to as "cross counts".

The O2 sensor 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 O2 sensor signals, but uses TPS and MAP or MAF values to determine air/fuel ratio from a table built into memory. When ECM reads a voltage signal greater than .45 volt from the O2 sensor, ECM begins to alter commands to injector to produce either a leaner or richer mixture.

Once vehicle has entered "closed loop", a fault in the O2 circuit (cooled-down sensor or open or shorted O2 sensor circuit) is the only thing which can return vehicle to open loop. A problem in the O2 sensor circuit should set a related trouble code.

On 4.3L engines, O2 sensor uses an internal heating element. Heating element allows O2 sensor to warm more quickly, allowing fuel system to enter closed loop operation sooner. Heating element also prevents fuel system from re-entering open loop operation, which would be a normal response to prolonged idling.