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 mega Ohm digital voltmeter, this may display as low as 350 mV. The oxygen sensor varies the voltage within a range of about 800 mV when the exhaust is rich, down through about 50 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 fuel trim, fuel injector circuit, Misfire, EVAP, EGR, TP, IAT, MAP, ECT, CKP, or MAF sensor DTCs present.
^ System voltage is greater than 9 volts.
^ Closed loop commanded air/fuel ratio is between 14.4 and 14.9.
^ Throttle angle is between 3 percent and 40 percent.

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 50 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the Malfunction Indicator Lamp 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 that the diagnostic runs and does not fail.
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A History DTC clears after forty consecutive warm-up cycles, if this or any Other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.
^ Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Diagnostic Aids

Notice: Use the Connector Test Adapter Kit J 35616-A for any test that requires probing the following items:
^ The PCM harness connectors
^ The electrical center fuse/relay cavities
^ The component terminals
^ The component harness connector Using this kit will prevent damage caused by the improper probing of connector terminals. 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 Pressure Test. Fuel System Pressure Test
^ Fuel injector(s) Refer to Fuel Inj Coil Test - ECT Between 10-35 Degrees C or Fuel Inj Coil Test Component Tests and General Diagnostics
- ECT Outside 10-35 Degrees C.
^ Vacuum leaks Check for the following conditions:
- Disconnected or damaged vacuum hoses.
- Incorrectly installed or malfunctioning crankcase ventilation valve.
- Vacuum leaks at the Throttle body, EGR valve, and intake manifold mounting surfaces.
^ 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. Refer to Exhaust System Leak Diagnosis in Exhaust System.
^ 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
^ If none of the above conditions are present, replace the affected HO2S.

The Heated Oxygen Sensors require an air reference (outside air) for proper operation. The air reference is supplied through the HO2S wiring. Inspect the HO2S wires and connections for breaks or contamination. Do not use solder to repair HO2S wiring. Solder will obstruct the air path.

Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
3. In order to determine if the engine is lean during power enrichment operate the vehicle under higher Throttle/acceleration conditions while monitoring scan tool HO2S voltage and the power enrichment parameter. A lean condition will cause HO2S voltage to be less than 598 mV during power enrichment.