P0138

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heated Oxygen Sensor (HO2S) signal circuit and the HO2S low reference circuit. When measured with a 10 megohm digital voltmeter, this voltage may display as low as 350 mV. The oxygen sensor signal varies from about 800 mV when the exhaust is rich, to about 50 mV when the exhaust is lean. The PCM constantly monitors the HO2S signal during Closed Loop operation. The PCM then compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary. If the HO2S 2 voltage remains excessively high for an extended period of time, DTC P0138 will set.

CONDITIONS FOR RUNNING THE DTC
^ DTC's P0101, P0102, P0103, P0106, P0107, P0108, P0111, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0200, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P1106, P1107, P1111, P1112, P1114, P1115, P1121, P1122, or P1441 are not set.
^ The system voltage is more than 9 volts.
^ The Closed Loop commanded air-to-fuel ratio is between 14.4-14.9.
^ The throttle angle is between 3-40 percent.

CONDITIONS FOR SETTING THE DTC
The HO2S 2 signal voltage remains above 999 mV during normal Closed Loop operation.

OR

HO2S 2 signal voltage remains above 200 mV during Deceleration Fuel Mode operation.

Either condition is present for up to 50 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM turns the MIL OFF after 3 consecutive drive trips during which the diagnostic runs and passes.
^ A last test failed, or the current DTC, clears when the diagnostic runs and passes.
^ A History DTC clears after 40 consecutive warm-up cycles, if no other emission related diagnostic failures are reported.
^ Use a scan tool in order to clear the MIL diagnostic trouble code.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM) Description, Clearing Diagnostic Trouble Codes.

DIAGNOSTIC AIDS

NOTE: Refer to Connector Test Adapter Kit J 35616-A Notice in Service Precautions

Check for the following components and conditions:
^ The fuel pressure - An excessively rich fuel mixture can cause a DTC P0138 to set.
^ The fuel injectors - Refer to Fuel Injector Coil Test. Component Tests and General Diagnostics
^ The Evaporative Emissions (EVAP) canister purge - Check for fuel saturation. If full of fuel, check the canister control and hoses. Refer to Evaporative Emission (EVAP) Control System Description.
^ The fuel pressure regulator - Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for the presence of fuel. If fuel is present, replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement.
^ The TP sensor - An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. The Throttle Angle displayed on a scan tool should increase steadily from 0-100 percent when opening the throttle. If this does not occur, check the TP sensor for loose mounting screws. If the TP mounting screws are OK, replace the TP sensor. Refer to Throttle Position (TP) Sensor Replacement.
^ A shorted Heated Oxygen Sensor (HO2S) - If the HO2S is internally shorted the HO2S voltage displayed on the scan tool will be over 1 volt. Try disconnecting the affected HO2S and jumper the HO2S low reference circuit to ground with the ignition ON and the engine OFF. If the displayed HO2S voltage changes from more than 1000 mV to approximately 450 mV, replace the HO2S. Silicon contamination of the HO2S can also cause a high HO2S voltage to be indicated. Silicon contamination is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is evident, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) 2 Replacement.
^ Open HO2S high signal or low reference circuits or malfunctioning HO2S - A poor connection or open in the HO2S high signal or low reference circuits can cause the DTC to set during deceleration fuel mode. An HO2S which is malfunctioning and not allowing a full voltage swing between the rich and lean thresholds can also cause this condition. Operate the vehicle while monitoring the HO2S voltage with a scan tool. If the HO2S voltage is limited within a range between 300 mV to 600 mV, check the HO2S high signal and low reference circuits wiring and associated terminal connections. If the wiring and connections are OK, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) 2 Replacement.

If the problem is intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-3:

Steps 4-11:

The numbers below refer to the step numbers on the diagnostic table.
3. In order to determine if the engine is rich during Deceleration Fuel Cut-Off (DFCO) operate the vehicle at highway speed and release the accelerator pedal, allowing the vehicle to coast in gear. Monitor the scan tool HO2S voltage and the DFCO parameter. A rich condition will cause the HO2S voltage to be more than 468 mV during DFCO.

An HO2S that is contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual causes of silica contamination are the use of an incorrect silicon RTV engine gasket material, the use of silicon based sprays or fluids within the engine, or coolant consumption. Phosphorus contamination also leaves a white powdery coating on the HO2S. Phosphorus contamination results from oil consumption. If the cause of the contamination is not corrected, the replacement HO2S will also become contaminated.
6. This step checks the HO2S high signal circuit for a short to voltage.
8. An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual causes of silica contamination are the use of incorrect silicon RTV engine gasket material, the use of silicon based sprays or fluids within the engine, or coolant consumption. Phosphorus contamination also leaves a white powdery coating on the HO2S. Phosphorus contamination comes from oil consumption. If the cause of the contamination is not corrected, the replacement HO2S will also become contaminated.