Circuit Description
The powertrain control module (PCM) supplies a bias voltage of about 450 mV between the heated oxygen sensor (HO2S) high signal circuits and the low signal circuits. When measured with a 10-megaohm DMM, this voltage 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 the 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 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, 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 during normal closed loop operation.
OR
| • | The HO2S 2 signal voltage remains above 200 mV during deceleration fuel mode operation. |
| • | For VIN K , the condition is present for up to 4 minutes. |
| • | For VIN 1, the condition is present for up to 6 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
Inspect for the following conditions:
| • | The fuel pressure--inspect for an excessively rich fuel mixture. |
| • | The fuel injectors--Refer to Fuel Injector Solenoid Coil Test . |
| • | The evaporative emissions (EVAP) canister purge--Inspect for fuel saturation. If the canister is full of fuel, inspect the canister control and inspect the hoses. |
| • | The mass air flow (MAF) sensor--Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | The fuel pressure regulator--Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for 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. The throttle angle displayed on a scan tool should increase steadily from 0-100 percent when you open 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 . |
| • | A shorted heated oxygen sensor (HO2S)--If the HO2S is internally shorted, the HO2S voltage displayed on the scan tool will be more than 1 volt. disconnect the affected HO2S, and jumper the HO2S low signal circuit to ground with the key ON, and with the engine not running. If the displayed HO2S voltage changes from more than 1000 mV to approximately 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 . |
| • | An open HO2S high signal or low signal circuit, or a malfunctioning HO2S--A poor connection or open in the HO2S high or low signal 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 300-600 mV, check the HO2S high signal circuit wiring, the low signal circuit wiring, and the associated terminal connections. If the wiring and connections are OK, replace the affected HO2S. Refer to Heated Oxygen Sensor 1 Replacement . |
If the condition is intermittent, refer to Intermittent Conditions .
Test Description
The number below refers to the step number on the diagnostic table:
Step | Action | Values | 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 H02S voltage parameters. If all parameters remain below the specified value , refer to Diagnostic Aids.
Does the voltage measure more than the specified value? | 999 mV | Go to Step 4 | Go to Step 3 |
Does the DTC set? | -- | Go to Step 4 | Go to Diagnostic Aids | |
4 | Disconnect the HO2S 2, and jumper the HO2S low signal circuit to ground. Does the voltage measure near the specified value? | 450 mV | Go to Diagnostic Aids | Go to Step 5 |
5 |
Does the voltage measure more than the specified value? | 999 mV | Go to Step 6 | Go to Step 7 |
6 | Repair any 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, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
