P0131

Refer to Underhood HO2S in Engine Controls Schematics.

Diagnostic Chart:

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 DMM, 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
^ Active TP, MAP, IAT, ECT, MAF sensor, fuel injector circuit, EVAP, AIR DTCs are not present.
^ Closed loop commanded air/fuel ratio is between 13 and 16.5.
^ Throttle angle is between 3 percent and 40 percent.
^ AIR pump commanded OFF.
^ Ignition voltage is between 9.0 and 18.0 volts.

CONDITIONS FOR SETTING THE DTC
HO2S 1 signal voltage remains below 175 mV during normal closed loop operation for more than 50 seconds.
OR
HO2S 1 signal voltage remains below 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 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.
^ If the misfire is determined to be catalyst damaging, the PCM will flash the MIL.

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 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-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, a DTC P0131 may be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Lean injectors-Perform Injector Balance Test. Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ Vacuum leaks-Inspect 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.
^ 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. Initial Inspection and Diagnostic Overview
^ 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

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 (TP angle above 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.