P0131

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heated Oxygen Sensor (HO2S) high and low signal circuits. When measured with a 10 megaohm Digital Multimeter (DMM), this may display as low as 320 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 set.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
^ System voltage is greater than 9 volts.
^ Closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ Throttle angle is between 3 percent and 35 percent.

CONDITIONS FOR SETTING THE DTC
The HO2S 1 signal voltage remains below 175 mV during normal closed loop operation for longer than 45 seconds.

OR

The HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation for longer than 55 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ First failure:
- The MIL is not turned ON.
- A DTC is stored in memory under Failed Last Test.
- The Failure Records are stored.
^ Second consecutive drive cycle with a failure:
- The MIL is turned ON.
- A DTC is stored in memory under history.
- The Freeze Frame data is stored.
- The Failure Records are stored.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after the third consecutive trip the diagnostic has run and passed.
^ The history DTC will clear if the fault conditions have not been detected for 40 warm-up cycles.
^ A scan tool can be used to clear DTC information.

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. Refer to Ground Distribution Schematics.
^ 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
^ Fuel injectors. Refer to Fuel Injector Coil Test. Component Tests and General Diagnostics
^ Vacuum leaks. Check for the following conditions:
- Disconnected or damaged vacuum hoses.
Refer to Emission Hose Routing Diagram.
- Incorrectly installed or malfunctioning crankcase ventilation valve.
Refer to Crankcase Ventilation System Inspection/Diagnosis in engine. Positive Crankcase Ventilation
- 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 Leakage. Testing and Inspection
^ Mass air flow (MAF) sensor. Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the 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 (Without Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (With Special Tool). Component Tests and General Diagnostics Component Tests and General Diagnostics
^ If none of the above conditions are present, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) 1 Replacement.

If the DTC cannot be duplicated and is determined to be intermittent, reviewing the Freeze Frame/Fail Records can be useful in determining when the DTC was last set. Also refer to Testing for Intermittent and Poor Connections in Diagrams.


TEST DESCRIPTION

Steps 1-9:

The number below refers to the step number 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 Freeze Frame/Fail Records conditions are being duplicated, an intermittent condition is indicated. Review the Freeze Frame/Fail 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.