P0138

Diagnostic Chart:

Schematic:

When the Heated Oxygen Sensor (HO2S) sensor 2 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.

DTC PARAMETERS
DTC P0138 will set if the HO2S sensor 2 voltage is greater than 1042 mV for 50 seconds.
^ Throttle position is between 5% and 50% for 4 seconds.
^ Engine coolant temperature is greater than 40°C (104°F) for 140 seconds.
^ Air flow is greater than 5.5 grams per second for 140 seconds.
^ DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0171, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0336, P0440, P0442, P0446, P0506, P0507, P0601, P0602 or P1441 are not set.

DIAGNOSTIC AIDS
If a front O2S DTC and a rear HO2S sensor 2 DTC are set, diagnose the front O2S circuit first. A diagnosed front O2S DTC may lead to a mechanical or fuel system failure which set the rear HO2S sensor 2 DTC.

IMPORTANT: Do not apply grease or any material in the HO2S sensor 2 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 atmosphere.