Oxygen (O2) Sensor

The O2 sensor is mounted in the exhaust system where it 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 signal line. This is referred to as "cross counts".

The O2 sensor will 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 will function in "open loop" mode and ECM will not make air/fuel adjustments based upon O2 sensor signals but will use TPS and MAP values to determine air/fuel ratio from a table built into memory. When ECM reads a voltage signal of more than .45 volts from the O2 sensor, ECM will begin to alter commands to injector or M/C solenoid to produce either a leaner or richer mixture. Once vehicle has entered "closed loop", a fault in the O2 circuit (cooled-down, open or shorted O2 sensor) is the only thing which can return it to "open loop". A problem in the O2 sensor circuit should set Code 13, 44 or 45.