P0138

Diagnostic Chart:

Schematic:

CIRCUIT DESCRIPTION
The PCM supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 10 megohm DMM this may display as low as 320 mV. The oxygen sensor varies the voltage within a range of about 1000 mV when the exhaust is rich, down through about 10 mV when exhaust is lean. The PCM constantly monitors the HO2S signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing 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
^ No active TP sensor, EVAP system, misfire, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, EGR Pintle Position, ECT sensor, CKP sensor, or MAF sensor DTCs present.
^ Closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ Throttle angle is between 3 percent and 40 percent.

CONDITIONS FOR SETTING THE DTC
^ 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.
^ The condition is present for up to 4 minutes.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.
^ If the misfire is determined to be catalyst damaging, the PCM will flash the MIL.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
^ The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

DIAGNOSTIC AIDS
Inspect for the following conditions:
^ Fuel pressure: An excessively rich fuel mixture. Refer to Fuel System Pressure Test.
^ Fuel injectors: Refer to Fuel Injector Coil Test Engine Coolant Temperature (ECT) Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Outside 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Evaporative emissions (EVAP) canister purge: Inspect for fuel saturation. If full of fuel, inspect canister control and hoses.
Refer to EVAP Control System Diagnosis.
^ Mass Air Flow sensor: Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace MAF sensor. Refer to MAF Sensor Replacement.
^ Fuel Pressure Regulator: Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to regulator for the presence of fuel. If fuel is present, replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement.
^ TP Sensor: An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. Throttle Angle displayed on a scan tool should increase steadily from 0 percent to 100 percent when opening the throttle. If not, check the TP sensor for loose mounting screws. If the TP mounting screws are OK, replace the TP sensor. Refer to TP Sensor Replacement.
^ 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 circuit to ground with the key ON, engine not running. If the displayed HO2S voltage changes from over 1000 mV to around 450 mV, replace the HO2S. Silicon contamination of the HO2S can also cause a high HO2S voltage to be indicated. This condition 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.
^ Open HO2S Signal or Low Circuit or Malfunctioning HO2S: A poor connection or open in the HO2S signal or low circuit 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 300mV to 600mV, check the HO2S signal and low circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the affected HO2S.

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams for diagnosis and repair.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Diagrams for diagnosis and repair. Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
Numbers below refer to step numbers on the Diagnostic Table:
3. DTC P0138 failing during deceleration fuel mode operation (vehicle speed above 40 km/h / 25 mph, TP angle below 3 percent) may indicate a condition described in Diagnostic Aids.
7. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.