P0131

Schematic:

CIRCUIT DESCRIPTION
The PCM supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 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 voltage remains excessively low for an extended period of time, this DTC will be set.

CONDITIONS FOR RUNNING THE DTC
^ No Misfire, CKP, Fuel injector, MAF, TP, EVAP, IAT, MAP, Fuel trim, EGR or ECT DTCs are present.
^ The vehicle is in closed loop.
^ The Air/Fuel Ratio is more than or equal to 14.5 but less than or equal to 14.8
^ Throttle angle is between 5 percent and 40 percent

CONDITIONS FOR SETTING THE DTC
HO2S signal voltage remains less than 175 mV during normal closed loop operation
OR
HO2S signal voltage remains less than 600 mV during power enrichment.

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 the MIL OFF 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 a scan tool.

DIAGNOSTIC AIDS
Inspect for the following:
^ Throttle body inlet screen blockage
^ 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 HO2S DTC may set. Refer to Fuel System Pressure Test. Component Tests and General Diagnostics
^ Fuel injectors contaminated or faulty - Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ 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. Inspect 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 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 for procedure to test for fuel contamination. Testing and Inspection

If none of the conditions are present, replace the affected HO2S.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION

Diagnostic Chart:

The numbers below refer to step numbers on the diagnostic table:
3. This DTC failing during power enrichment operation (TP angle above 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.
6. 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.