Oxygen (O2) Sensor

The oxygen sensor is mounted in the exhaust system where it monitors oxygen content of exhaust gases. Two 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%) 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 oxygen sensor signal line. This is referred to as "cross counts."

The oxygen sensor will not function properly (produce voltage) until its temperature reaches approximately 600°F (316°C). On 3.1L California "W" Body and 4.3L Caprice, oxygen sensor is equipped with a sensor heating element. 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).

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

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