P0131

Circuit Description
The powertrain control module (PCM) supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 10 meg ohm 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 is set.

Conditions for Running the DTC
^ None of the following DTCs are present:
- Throttle position (TP) sensor DTCs
- Evaporative emissions (EVAP) DTCs
- Intake air temperature (IAT) sensor DTCs
- Manifold absolute pressure (MAP) sensor DTCs
- Exhaust gas recirculation (EGR) pintle position DTCs
- Engine coolant temperature (ECT) sensor DTCs
- Crankshaft position (CKP) sensor DTCs
- Mass air flow (MAF) sensor DTCs
- Misfire DTCs
- Fuel trim DTCs
- Fuel injector circuit DTCs
^ The system voltage is more than 9 volts.
^ The closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ The Throttle angle is between 3% and 40%.

Conditions for Setting the DTC
^ The HO2S 1 signal voltage remains below 175 mV during normal closed loop operation.
OR
^ The HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation.
^ Either condition is present for up to 50 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has run and failed.
^ The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed.
^ The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids
Inspect for the following conditions:
^ Inspect the heated oxygen sensor wiring. The sensor pigtail may be routed incorrectly and contacting the exhaust system.
^ Inspect for faulty PCM to engine block grounds.
^ Test the fuel pressure. The system goes lean if the pressure is too low. The PCM may compensate for some decrease. However, if the fuel pressure is too low, a DTC P0131 may be set. Refer to Fuel System Pressure Test. Fuel System Pressure Test
^ Test the fuel injector(s) Refer to Fuel Inj Coil Test Component Tests and General Diagnostics
- ECT Between 10-35 Degrees C or Fuel In] Coil Test - ECT Outside 10-35 Degrees C.
^ Inspect for the following vacuum leak conditions:
- Any disconnected or damaged vacuum hoses.
- An incorrectly installed or malfunctioning crankcase ventilation valve.
- Inspect for vacuum leaks at the Throttle body, the EGR valve, and the intake manifold mounting surfaces.
^ Inspect for 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.
^ Disconnect the MAF sensor and note if the lean condition is corrected. If the condition is corrected by disconnecting the MAF sensor, replace the MAF sensor.
^ Test for fuel contamination. Water, even in small amounts, may be delivered to the fuel injectors. The water may cause a lean exhaust to be indicated. Excessive alcohol in the fuel may also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. Testing and Inspection
^ If none of the above conditions are present, replace the affected HO2S.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
3. 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. Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
7. 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.