P0131

Diagnostic Chart:

Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of approximately 450 mV between the Heated Oxygen Sensor (HO2S) signal and the low circuits. When measured with a 10 megaohm Digital Voltmeter (DVM), this may display as low as 350 mV. The oxygen sensor varies the voltage from approximately 1000 mV when the exhaust is rich, to approximately 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 the injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended time, DTC P0131 will set.

CONDITIONS FOR RUNNING THE DTC
^ The system voltage is between 9.0 and 18.0 volts.
^ There are no active Diagnostic Trouble Codes (DTCs) present:
- Throttle Position (TP)
- Manifold Absolute Pressure (MAP)
- Intake Air Temperature (IAT)
- Engine Coolant Temperature (ECT)
- Mass Air Flow (MAF)
- Fuel injector circuit
- Evaporative Emission (EVAP)
- AIR
^ The closed loop commanded air-to-fuel ratio is 13.0-16.5.
^ The AIR pump is commanded OFF.
^ The throttle angle is between 3 percent and 40 percent.

CONDITIONS FOR SETTING THE DTC
HO2S 1 signal voltage remains less than 175 mV during normal closed loop operation for more than 50 seconds.

OR

HO2S 1 signal voltage remains less than 600 mV during power enrichment mode fuel control operation for more than 10 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 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 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 a scan tool.

DIAGNOSTIC AIDS
Inspect for:
^ Heated oxygen sensor wiring-The sensor pigtail may be routed incorrectly and contacting the exhaust system. Refer to Wiring Repairs in Diagrams.
^ Faulty PCM to engine block grounds - Refer to Ground Distribution Schematics in Diagrams.
^ 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, DTC P0131 may be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Lean injector - Perform an 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, the throttle body, the EGR system, and the crankcase ventilation system. Refer to Visual and Physical Check in Symptoms. Initial Inspection and Diagnostic Overview
^ 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.
^ 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. Component Tests and General Diagnostics

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

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.
^ Damaged harness - Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault.
^ PCM and engine grounds for clean and secure connections

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

TEST DESCRIPTION
The numbers below refer to step numbers on the diagnostic table.
3. This DTC failing during power enrichment operation, with a TP angle that is more than 60 percent, may indicate a condition described in Diagnostic Aids. If this DTC test passes while the Failure Records conditions are being duplicated, an intermittent condition is indicated. Review the Failure 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.