The heated rear oxygen sensor (HO2S) consists of a zirconia electrolyte between 2 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. Refer to DTC P0141 for heater circuit operation.

Important: The HO2S signal is continuously checked once the above conditions have been met for 0.5 seconds.

DTC P0137 will set if the HO2S voltage is lower than 25 mV for 100 seconds

OR

HO2S voltage is less than 291 mV in power enrichment (PE) mode for 30 seconds when:

Important: Do not apply grease or any material in the HO2S connector or alter the wiring in any way. The wires carry reference air to the sensor used to determine the difference in oxygen from the exhaust gas to the atmosphere.

Possible Causes

If a front O2S DTC and a rear HO2S DTC are set, diagnose the front O2S fault first. A diagnosed from O2S DTC may lead to a mechanical or fuel system failure which set the rear HO2S DTC.