P0131

Circuit Description

The PCM supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 10 megaohm 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

^ No active TP, MAP, IAT, ECT, MAF, CKP sensor, misfire, fuel injector circuit, EVAP, Fuel trim, EGR DTCs present.
^ Closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ Throttle angle is between 3% and 40%.

Conditions for Setting the DTC

HO2S 1 signal voltage remains below 175 mV during normal closed loop operation.
OR
^ HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation.
^ Either condition for up to 5 seconds.

Action Taken When the DTC Sets

^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed.
^ If equipped with traction control, the PCM will command the Electronic Brake and Traction Control Module (EBTCM) via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail 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

Check for the following conditions.

^ Heated oxygen sensor wiring. The sensor pigtail may be routed incorrectly and contacting the exhaust system.
^ Poor PCM to engine block grounds.
^ 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 Diagnosis. Component Tests and General Diagnostics
^ Lean injector(s). Perform Injector Balance Test. Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system. Refer to Visual/Physical Check in Symptoms. Symptom Related Diagnostic Procedures
^ 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.
^ 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 Diagnosis. Testing and Inspection

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

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

The numbers below refer to the step numbers on the Diagnostic Table:

3. DTC P0131 failing during power enrichment operation may indicate a condition described in the Diagnostic Aids above. 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.