Circuit Operation

The heated rear oxygen sensor (HO2S-2) consists of a zirconia electrolyte between two platinum plates. When the sensor reaches approximately 318°C (600°F) it becomes an electrical source that responds to the oxygen content in the exhaust. The PCM produces a bias voltage of approximately 450 millivolts on the oxygen sensor circuit. When the sensor is cold its internal resistance is extremely high, therefore, the PCM recognizes the bias as an open circuit. As the sensor warms up, the internal resistance decreases. When the sensor reaches approximately 318°C (600°F), it starts producing a voltage based on the oxygen content in the exhaust stream.


The heated oxygen sensor (HO2S-2) is mounted behind the catalytic converter. This sensor is used to determine the oxygen storage capability of the catalytic converter. Oxygen storage capability is used as a measure of converter condition, and its ability to perform its function. Due to the location of the rear HO2S, it requires additional heat to operate properly. This sensor incorporates a heater (12 ohms), powered by ignition voltage which allows the sensor to remain functional under most operating conditions. In order to determine that the sensor heater is functioning, its function can only be tested after what is considered a cold start. The PCM will monitor airflow and the time it takes for the sensor to become functional. If a start up is considered to be a hot restart the heater function cannot be tested due to the amount of heat stored by the catalytic converter. Also if airflow after start up is too high the sensor will not be tested due to the amount of heat generated by the engine making it difficult to accurately diagnose a heater malfunction.