P0131

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Engine Controls Schematics: Heated O2 Sensors:

CIRCUIT DESCRIPTION
The PCM supplies a bias voltage of about 450 mV between the HO2S signal circuit and the HO2S low circuit. When measured with a 10 megohm digital voltmeter, this voltage may display as low as 350 mV. The oxygen sensor signal varies from about 800 mV when the exhaust is rich, to about 50 mV when the 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 I voltage remains excessively low for an extended period of time, DTC P0131 will set.

CONDITIONS FOR RUNNING THE DTC
^ There are no active DTCs of any of the following types present:
^ Fuel trim
^ Fuel injector circuit Misfire
^ Evaporative Emissions System (EVAP)
^ Exhaust Gas Recirculation (EGR)
^ Throttle Position (TP)
^ Intake Air Temperature (IAT)
^ Manifold Absolute Pressure (MAP)
^ Engine Coolant Temperature (ECT)
^ Crankshaft Position (CKP)
^ Mass Air Flow (MAF)
^ The system voltage is more than 9 volts.
^ The closed loop commanded air-to-fuel ratio is between 14.4-14.9.
^ The throttle angle is between 3-40 percent.

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 of the above conditions is present for up to 50 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM turns the MIL OFF after three consecutive drive trips during which the diagnostic runs and passes.
^ A last test failed, or the current DTC, clears when the diagnostic runs and passes.
^ A History DTC clears after forty consecutive warm-up cycles, if no other emission related diagnostic failures are reported.
^ Use a scan tool in order to clear the MIL diagnostic trouble code.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM)Description, Clearing Diagnostic Trouble Codes.

DIAGNOSTIC AIDS

NOTE: Refer to Connector Test Adapter Kit J 35616-A Notice in Service Precautions.

Check for the following conditions:
^ Incorrectly routed Heated Oxygen Sensor wiring - The sensor pigtail may be routed incorrectly and contacting the exhaust system.
^ Faulty PCM to engine block grounds - Refer to Ground Distribution Schematics.
^ Fuel pressure that is too low - The system will go lean if the fuel 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 Pressure Test Fuel System Pressure Test
^ Problems with the fuel injectors - Refer to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Outside 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Vacuum leaks - Check for the following conditions:
- Disconnected or damaged vacuum hoses - Refer to Emission Hose Routing Diagram (Without Secondary Air Injection (AIR) or Emission Hose Routing Diagram (With Secondary Air Injection (AIR)) or Emission Hose Routing Diagram (Without Secondary Air Injection (AIR) or Emission Hose Routing Diagram (With Secondary Air Injection (AIR))
- An incorrectly installed or malfunctioning crankcase ventilation valve - Refer to Crankcase Ventilation System Diagnosis. Component Tests and General Diagnostics
- Leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces.
^ Exhaust leaks - An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, making the system appear lean. Check for exhaust leaks that may be producing a false lean condition. Refer to Inspection Procedure in Engine Exhaust.
^ Fuel contamination - Even small amounts of water in the fuel injectors 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. Refer to Heated Oxygen Sensor (HO2S) Replacement

If the problem is intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
3. In order to determine if the engine is lean during power enrichment, operate the vehicle under higher throttle, or higher acceleration, conditions, Monitor the scan tool HO2S voltage and the power enrichment parameter at the higher acceleration. A lean condition will cause the HO2S voltage to be less than 598 mV during power enrichment.