For 1990-2009 cars

### 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.

• | Clear the MIL and the DTC with a scan tool.

### Diagnostic Aids

Inspect for the following conditions:

• | The fuel pressure--An excessively rich fuel mixture

• | Rich injectors--Refer to Fuel Injector Solenoid Coil Test .

• | 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 Airflow 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 postion (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 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 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 1 Replacement or Heated Oxygen Sensor 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 1 Replacement or Heated Oxygen Sensor 2 Replacement .

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 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.

1. DTC P0138 failing 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.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Important: During this step, observe all HO2S voltage parameters. If all parameters remain below the specified value refer to Diagnostic Aids.

1. Engine at operating temperature.
2. Operate vehicle within Conditions for Setting the DTC as specified in the supporting text.
3. Monitor the HO2S 2 voltage with a scan tool.

Does the voltage measure more than the specified value?

999 mV

Go to Step 4

Go to Step 3

3 |

1. Turn ON the ignition, with the engine OFF.
2. Review and record scan tool Failure Records data.
3. Operate the vehicle within the Failure Records conditions.
4. With a scan tool, observe Specific DTC info for DTC P0138 until the DTC P0138 test runs.

Does the DTC reset?

--

Go to Step 4

Go to Diagnostic Aids

4 | Disconnect HO2S 2 and jumper HO2S low reference circuit to a good ground. Does the voltage measure near the specified value? | 450 mV | Go to Step 8 | Go to Step 5
5 |

1. Turn OFF the ignition.
2. Disconnect PCM connector.
3. Turn ON the ignition, with the engine OFF.
4. Using a DMM, measure voltage between the HO2S 2 high signal circuit at PCM harness connector and ground.

Does the voltage measure more than the specified value?

999 mV

Go to Step 6

Go to Step 7

6 | Repair short to voltage in the HO2S 2 high signal circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 9 | --
7 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 9 | --
8 | Replace the affected HO2S sensor. Refer to Heated Oxygen Sensor 1 Replacement or Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | -- | Go to Step 9 | --
9 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 10

Go to Step 2

10 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK