Circuit Description
The PCM supplies a bias voltage of approximately 450 mV between the HO2S signal and the low circuits. When measured with a 10-megaohm DMM, the bias voltage may display as low as 320 mV. The oxygen sensor varies the voltage from approximately 1000 mV when the exhaust is rich, to approximately 100 mV when exhaust is lean. The PCM monitors the HO2S signal during closed loop operation. The PCM compensates for a rich or a lean condition by decreasing or increasing the injector pulse width. If the HO2S 1 voltage remains excessively low for an extended time, DTC P0131 will set.
Conditions for Running the DTC
| • | None of the following DTCs are active: |
| - | TP sensor |
| - | EVAP system sensor |
| - | Misfire sensor |
| - | IAT sensor |
| - | MAP sensor |
| - | Fuel trim sensor |
| - | Fuel injector circuit sensor |
| - | ERG pintle position sensor |
| - | ECT sensor |
| - | CKP sensor |
| - | MAF sensor |
| • | The system voltage is more than 9 volts. |
| • | The closed loop commanded air-to-fuel ratio is between 14:5 and 14:8. |
| • | The throttle angle is between 3 percent and 40 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 the power enrichment mode of the fuel control operation for longer than 55 seconds. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Faulty heated oxygen sensor wiring -- The sensor pigtail may be routed incorrectly and may be contacting the exhaust system. |
| • | Faulty PCM to engine block grounds -- Refer to Ground Distribution Schematics in Wiring Systems. |
| • | Faulty fuel pressure -- The system will go lean if the fuel pressure is too low. The PCM can compensate for some decrease. If the fuel pressure is too low, a DTC P0131 may set. Refer to Fuel System Pressure Test (VIN K) or refer to Fuel System Pressure Test (VIN 1) . |
| • | Vacuum leaks -- Check the throttle body, the EGR valve, and the intake manifold mounting surfaces for the following conditions: |
| - | Disconnected or damaged vacuum hoses -- Refer to Emission Hose Routing Diagram . |
| - | An incorrectly-installed or a malfunctioning crankcase ventilation valve -- Refer to Crankcase Ventilation System Inspection . |
| • | Exhaust leaks -- An exhaust leak may cause the outside air to be pulled into the exhaust gas stream past the HO2S. This condition may cause the exhaust to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust Leakage - Not OBD II . |
| • | A faulty MAF sensor -- Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. |
| • | Contaminated fuel -- 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 Replacement . |
If you cannot duplicate the DTC and you determine that the condition is intermittent, review the Failure Records in order to determine when the DTC last set. Also refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
-
If this DTC fails during power enrichment operation, indicated by the TP angle above 60 percent, may indicate a condition described in Diagnostic Aids. If this DTC test passes while the Failure Records conditions are duplicated, an intermittent condition is indicated. Review the Failure Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
-
When the PCM is replaced, you must program the new PCM.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important: During this step, observe all H02S voltage parameters. If all parameters remain below the specified value, refer to Diagnostic Aids.
Does the HO2S voltage remain below the specified value? | 400 mV | ||
Operate the vehicle within the Failure Records conditions. Does the scan tool indicate this DTC failed this ignition? | -- | Go to Diagnostic Aids | ||
4 |
Does scan tool indicate that the HO2S voltage less than the specified value? | 400 mV | ||
5 | Test the HO2S signal circuit for a short to ground, or for a short to the sensor ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
|
Important: You must program the replacement PCM. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | -- | ||
7 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement . Did you complete the repair? | -- | -- | |
8 |
Does the DTC reset? | -- | System OK |
