Circuit Description
The PCM supplies a bias voltage of about 450 mV 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 Bank 1 HO2S 1 voltage remains excessively low for an extended period of time, DTC P0131 will be set.
Conditions for Setting the DTC
| • | No active transmission, TP sensor, EVAP system, misfire, EGR, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, EVAP solenoid ODM, ECT sensor, or MAF sensor DTC(s) present. |
| • | Closed loop commanded air/fuel ratio is between 14.5 and 14.8 |
| • | Throttle angle is between 3% and 40% |
| • | Bank 1 HO2S 1 signal voltage remains below 175 mV during normal closed loop operation |
OR
| • | Bank 1 HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation. |
| • | Either condition must be present for at least 5 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 and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been 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 the scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Heated oxygen sensor wiring damaged -- The sensor pigtail may be routed incorrectly and contacting the exhaust system. |
| • | Poor PCM to engine block grounds |
| • | Low 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 Pressure Test . |
| • | Fuel injector(s) contaminated or faulty -- Refer to Fuel Injector Balance Test |
| • | 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. |
| • | Faulty MAF sensor -- Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace 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 Test for procedure to check for fuel contamination. |
If none of the above conditions are present, replace the affected HO2S.
Test Description
Number(s) below refer to step number(s) on the Diagnostic Table:
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DTC P0131 failing during power enrichment operation (TP angle above 60%) may indicate a condition described in Diagnostic Aids. If the DTC P0131 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.
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This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 |
Does the Bank 1 HO2S 1 voltage remain below the specified value? | 300 mV | ||
Note test result; does scan tool indicate DTC P0131 failed this ignition? | -- | Go to Diagnostic Aids | ||
4 | Disconnect Bank 1 HO2S 1 and jumper the Bank 1 HO2S 1 low circuit (PCM side) to ground. Does scan tool indicate Bank 1 HO2S 1 voltage near the specified value? | 450 mV | Go to Diagnostic Aids | |
5 | Turn off the ignition, disconnect the PCM, and check the Bank 1 HO2S 1 signal circuit for a short to ground or a short to the sensor ground circuit. Is the Bank 1 HO2S 1 signal circuit shorted? | -- | ||
6 | Repair the Bank 1 HO2S 1 signal circuit. Refer to Heated Oxygen Sensor (O2S) Repair in Electrical Diagnosis. Is action complete? | -- | -- | |
Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||
8 |
Note test result; does scan tool indicate DTC P0131 failed this ignition? | -- | System OK | |
