P0138
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 digital voltmeter, 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 be set.
CONDITIONS FOR RUNNING THE DTC
None of the following active DTCs are present:
- Throttle Position (TP) sensor
- Evaporative Emissions (EVAP) system
- Misfire
- Intake Air Temperature (IAT) sensor
- Manifold Absolute Pressure (MAP) sensor
- Fuel trim
- Fuel injector circuit
- EGR pintle position
- Engine Coolant Temperature (ECT) sensor
- Crankshaft Position (CKP) sensor
- Mass Air Flow (MAF) sensor
^ The closed loop commanded air/fuel ratio is between 14.5:1 and 14.8:1.
^ The throttle angle is between 3 percent and 40 percent.
CONDITIONS FOR SETTING THE DTC
^ The HO2S 2 signal voltage remains more than 999 mV.
^ The system is in closed loop operation.
or
^ The HO2S 2 signal voltage remains at more than 200 mV during deceleration fuel mode operation.
^ The condition is present for up to 8 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
Check for the following conditions:
^ The fuel pressure-An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Pressure Test. Fuel System Pressure Test
^ The 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
^ Check the Evaporative Emissions (EVAP) canister for fuel saturation. If the canister is full of fuel, check the canister control and hoses. Refer to EVAP Control System Diagnosis. Component Tests and General Diagnostics
^ Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace the MAF sensor.
^ 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.
^ 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.
^ 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. Refer to Heated Oxygen Sensor (HO2S) Replacement (Bank 1 Sensor 1) or Heated Oxygen Sensor (HO2S) Replacement (Bank 1 Sensor 2).
^ A faulty 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 300 mV to 600 mV, check the HO2S signal and low circuit wiring and associated terminal connections. Use a corresponding mating terminal to test for proper terminal tension. If the wiring and connections are OK, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) Replacement (Bank 1 Sensor 1) or Heated Oxygen Sensor (HO2S) Replacement (Bank 1 Sensor 2).
TEST DESCRIPTION
Diagnostic Chart:
The numbers below refer to the step numbers on the diagnostic table.
3. DTC P0138 failing during deceleration fuel mode operation (vehicle speed is more than 40 km/h (25 mph) and the TP angle is less than 3 percent) may indicate a condition described in Diagnostic Aids. If the DTC P0138 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
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.