P0138

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heater Oxygen Sensor (HO2S) high and low signal circuits. When measured with a 10-megohm Digital Multimeter (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 be set.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P01 18, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, P1441 are not set.
^ Closed loop commanded air/fuel ratio is between 14.5-14.8
^ Throttle angle is between 3-40 percent

CONDITIONS FOR SETTING THE DTC
^ HO2S 2 signal voltage remains above 999 mV.
^ System is in closed loop operation

OR

^ HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation
^ The condition is present for up to 5 minutes.

ACTION TAKEN WHEN THE DTC SETS
^ First failure:
- The MIL is not turned ON.
- A DTC is stored in memory under Failed Last Test.
- The Failure Records are stored.
^ Second consecutive drive cycle with a failure:
- The MIL is turned ON.
- A DTC is stored in memory under history.
- The Freeze Frame data is stored.
- The Failure Records are stored.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after the third consecutive trip the diagnostic has run and passed.
^ The history DTC will clear if the fault conditions have not been detected for 40 warm-up cycles.
^ A scan tool can be used to clear DTC information.

DIAGNOSTIC AIDS
Check for the following conditions:
^ Fuel pressure-An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Fuel injectors-Refer to Fuel Injector Coil Test. Component Tests and General Diagnostics
^ Evaporative Emissions (EVAP) canister purge-Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to DTC P0440. P0440
^ Mass Air Flow (MAF) sensor-Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace MAF sensor. Refer to Mass Air Flow (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 (VIN K) or Fuel Pressure Regulator Replacement (VIN 1).
^ Throttle Position (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 0-100 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 Throttle Position (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. Refer to Heated Oxygen Sensor (HO2S) 2 Replacement.
^ Open HO2S high signal or low signal circuits or malfunctioning HO2S-A poor connection or open in the HO2S high or low signal 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 300-600 mV, check the HO2S high and low signal circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) 2 Replacement.

TEST DESCRIPTION

Steps 1-9:

The number below refers to the step number on the diagnostic table.
3. DTC P0138 failing during deceleration fuel mode operation (vehicle speed above 16 km/h/10 mph, TP angle below 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 Failure 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.