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 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, diagnostic trouble code (DTC) P0138 will be set.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set. |
| • | The Closed Loop commanded air/fuel ratio is between 14.5-14.8 |
| • | The throttle angle is between 3-40 percent |
Conditions for Setting the DTC
| • | The HO2S 2 signal voltage remains above 999 mV. |
| • | The system is in Closed Loop operation |
OR
| • | The HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation |
| • | VIN K - The condition is present for up to 5 minutes. |
| VIN 1 - The condition is present for up to 8 minutes. |
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
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 . |
| • | Fuel injectors-Refer to Fuel Injector Solenoid Coil Test . |
| • | Evaporative emissions (EVAP) canister purge--Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to DTC 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 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 . |
| • | 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 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 1,000 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 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 2 Replacement . |
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 |
Important: During this step, observe all HO2S voltage parameters. If all parameters remain below the specified value refer to Diagnostic Aids.
Does the HO2S 2 voltage remain above the specified value? | 999 mV | Go to Step 4 | Go to Step 3 |
Does scan tool indicate DTC P0138 failed this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | |
4 | Disconnect HO2S 2 and jumper the HO2S low signal circuit to ground. Does scan tool indicate HO2S 2 voltage near the specified value? | 450 mV | Go to Diagnostic Aids | Go to Step 5 |
5 |
Does the voltage measure more than 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 8 | -- |
7 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 8 | -- |
8 |
Does the DTC run and pass? | -- | Go to Step 9 | Go to Step 2 |
9 | With a scan tool , observe the stored information in Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
