P0138

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heated Oxygen Sensor (HO2S) high signal and low signal circuits. When measured with a 10-megaohm DMM this may display as low as 320 mV. The oxygen sensor varies the voltage within a range of about 1,000 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
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
^ The Closed Loop commanded air/fuel ratio is between 12-16.5.
^ The throttle angle between 3-40 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.

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

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
Inspect for the following conditions:
^ The fuel pressure-An excessively rich fuel mixture
^ Rich injectors-Refer to Fuel injector Coil Test. Component Tests and General Diagnostics
^ Evaporative Emissions (EVAP) canister purge-Inspect for fuel saturation. If full of fuel, inspect canister control and hoses.
^ Mass air flow 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.
^ The 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 percent to 100 percent when opening the throttle. If not, inspect 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 circuit to ground with the key ON, engine OFF. If the displayed H028 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) 1 Replacement or Heated Oxygen Sensor (HO2S) 2 Replacement
^ An 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 300 mV to 600 mV, check the HO2S high signal and low signal circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the HO2S. Refer to Heated Oxygen Sensor (HO2S) I Replacement or Heated Oxygen Sensor (HO2S) 2 Replacement

TEST DESCRIPTION

Steps 1-10:

The numbers below refer to the step numbers on the diagnostic table.
3. DTC P0138 being set during deceleration fuel mode operation may indicate a condition described in the Diagnostic Aids above. If the DTC P0138 test passes while the Failure 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.
7. DTC P0138 tailing during deceleration fuel mode operation, with the vehicle speed above 40 km/h (25 mph), and with the TP angle below 3 percent may indicate a condition described in Diagnostic Aids.