Circuit Description
The powertrain control module (PCM) supplies a bias voltage of about 450mV between the HO2S signal and low circuits. When measured with a 10 megohm digital voltmeter, this may display as low as 350 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 be set.
Conditions for Running the DTC
| • | None of the following active DTCs are present: |
| - | Throttle position (TP) sensor |
| - | Evaporative emissions (EVAP) system |
| - | Misfire |
| - | Intake air temperature (IAT) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Fuel trim |
| - | Fuel injector circuit |
| - | EGR pintle position |
| - | Engine coolant temperature (ECT) sensor |
| - | Crankshaft position (CKP) sensor |
| - | Mass air flow (MAF) sensor |
| • | The system voltage is more than 9 volts. |
| • | The closed loop commanded air/fuel ratio is between 14.5:1 and 14.8:1. |
| • | The throttle angle is between 3 percent and 40 percent. |
Conditions for Setting the DTC
| • | The HO2S 1 signal voltage remains less than 175 mV during normal closed loop operation, or |
| • | the HO2S 1 signal voltage remains less than 600 mV during power enrichment mode fuel control operation. |
| • | Either condition is present for up to 60 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:
| • | The heated oxygen sensor pigtail may be routed incorrectly and contacting the exhaust system. |
| • | Any faulty PCM to engine block grounds, refer to Ground Distribution Schematics . |
| • | The fuel system will go lean if the fuel pressure is too low. The PCM can compensate for some decrease. However, if the fuel pressure is too low, a DTC P0131 may be set. Refer to the appropriate procedure: |
| • | Check for the following vacuum conditions: |
| - | Any disconnected or damaged vacuum hoses |
| - | Refer to Emission Hose Routing Diagram . |
| - | An incorrectly installed or malfunctioning crankcase ventilation valve, refer to Crankcase Ventilation System Inspection : |
| - | Vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces |
| • | 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 - Not OBD II in Engine Exhaust. |
| • | Check the throttle body inlet screen for damage or objects that may alter the airflow sample through the MAF sensor. If a problem is found, correct as necessary. Refer to Throttle Body Air Inlet Screen Replacement . |
| • | Disconnect the MAF sensor. If the lean condition is corrected with the MAF sensor disconnected, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | Check for 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 Replacement . |
Test Description
The numbers below refer to step numbers on the diagnostic table.
-
This DTC failing during power enrichment operation (TP angle more than 60 percent ) may indicate a condition described in Diagnostic Aids. If this DTC test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Review the Fail 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.
-
This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important: During this step, observe all HO2S voltage parameters. If all parameters remain below the specified value refer to Diagnostic Aids.
Does the voltage remain less than the specified value? | 400 mV | ||
Operate vehicle within the Failure Records conditions. Does the scan tool indicate this DTC failed this ignition? | -- | Go to Diagnostic Aids | ||
4 |
Is the voltage less than the specified value? | 400 mV | ||
5 | Test the HO2S 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? | -- | ||
|
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the repair? | -- | -- | ||
7 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement . Did you complete the repair? | -- | -- | |
8 |
Does the DTC reset? | -- | System OK |
