Circuit Description
The powertrain control module (PCM) supplies a bias voltage of about 450 mV between the heated oxygen sensor (HO2S) high and low signal circuits. When measured with a 10 megaohm digital multimeter (DMM), 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 100 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 1 voltage remains excessively low for an extended period of time, DTC P0131 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. |
| • | System voltage is greater than 9 volts. |
| • | Closed loop commanded air/fuel ratio is between 14.5 and 14.8. |
| • | Throttle angle is between 3 percent and 35 percent. |
Conditions for Setting the DTC
The HO2S 1 signal voltage remains below 175 mV during normal closed loop operation for longer than 45 seconds.
OR
The HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation for longer than 55 seconds.
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:
| • | Heated oxygen sensor wiring. The sensor pigtail may be routed incorrectly and contacting the exhaust system. |
| • | Poor PCM to engine block grounds. Refer to Ground Distribution Schematics . |
| • | Fuel pressure. The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0131 may be set. Refer to Fuel System Diagnosis . |
| • | Fuel injectors. Refer to Fuel Injector Solenoid Coil Test . |
| • | Vacuum leaks. Check for the following conditions: |
| - | Disconnected or damaged vacuum hoses. |
| - | Refer to Emission Hose Routing Diagram . |
| - | Incorrectly installed or malfunctioning crankcase ventilation valve. |
| - | Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical. |
| - | Vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces. |
| • | Exhaust leaks. An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust Leakage . |
| • | Mass air flow (MAF) sensor. Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the MAF sensor. |
| • | Fuel contamination. Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | If none of the above conditions are present, replace the affected HO2S. Refer to Heated Oxygen Sensor 1 Replacement . |
If the DTC cannot be duplicated and is determined to be intermittent, reviewing the Freeze Frame/Fail Records can be useful in determining when the DTC was last set. Also refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
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 voltage remain below the specified value? | 175 mV | Go to Step 4 | Go to Step 3 |
Operate vehicle within Freeze Frame/Fail Records conditions. Does the scan tool indicate this DTC failed this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | |
4 |
Does scan tool indicate HO2S voltage less than the specified value? | 400 mV | Go to Step 5 | Go to Step 7 |
5 | Test the HO2S high signal circuit for a short to ground or a short to the sensor ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 6 |
6 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 8 | -- |
7 | Replace the HO2S. Refer to Heated Oxygen Sensor 1 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 | |
