P0138

Circuit Description
The powertrain control module (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 the injector pulse width as necessary. If the HO2S 2 voltage remains excessively high for an extended period of time, DTC P0138 is set.

Conditions for Setting the DTC
None of the following DTCs are present:
- Throttle position (TP) sensor DTCs
- Evaporative emissions (EVAP) DTCs
- Intake air temperature (IAT) sensor DTCs
- Manifold absolute pressure (MAP) sensor DTCs
- Exhaust gas recirculation (EGR) pintle position DTCs
- Engine coolant temperature (ECT) sensor DTCs
- Crankshaft position (CKP) sensor DTCs
- Mass air flow (MAF) sensor DTCs
- Misfire DTCs
- Fuel trim DTCs
- Fuel injector circuit DTCs
^ The closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ The Throttle angle is between 3% and 40%.
^ The HO2S 2 signal voltage remains above 999 mV during normal closed loop operation.
OR
^ The HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation.
^ Either condition is present for up to 3 minutes.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has run and failed.
^ The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM turns the malfunction indicator lamp (MIL) off during the third consecutive trip in which the diagnostic has run and passed.
^ The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery teed.

Diagnostic Aids
Inspect or test for the following conditions:
^ Test the fuel pressure. An excessively rich fuel mixture may cause a DTC P0138 to be set. Refer to Fuel System Pressure Test. Fuel System Pressure Test
^ Test the fuel injectors. Refer to Fuel Inj Coil Test - ECT Between 10-35 Degrees C or Fuel Inj Coil Test - ECT Outside 10-35 Degrees C. Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Test the evaporative emissions (EVAP) canister purge. Inspect the canister for fuel saturation. If the canister is full of fuel, inspect the canister control and hoses.
Refer to EVAP Control System Diagnosis. Component Tests and General Diagnostics
^ Disconnect the MAF sensor and note if the rich condition is corrected. If disconnecting the sensor corrects the condition, replace the MAF sensor.
^ Test the fuel pressure regulator. Test for a leaking fuel pressure regulator diaphragm by inspecting the vacuum line to the regulator for the presence of fuel. If fuel is present, replace the fuel pressure regulator.
^ An intermittent TP sensor output causes 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% to 100% 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.
^ Test for a shorted heated oxygen sensor (HO2S). If the HO2S is internally shorted the HO2S voltage displayed on the scan tool is over 1 volt. Try disconnecting the affected HO2S and jumper the HO2S low circuit to ground with the key on and the 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 may 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.
^ Test for an open HO2S signal or low circuit or a malfunctioning HO2S. A faulty connection or an open in the HO2S signal or low circuit may 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 may 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 of 300 mV to 600 mV, inspect the HO2S signal and low circuit wiring and the associated terminal connections. If the wiring and connections are OK, replace the affected HO2S.

Test Description
Number(s) 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%) 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.