P0131

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heated Oxygen Sensor (HO2S) high signal and low signal circuits. When measured with a 10-megaohm DMM, this voltage may display as low as 320 mV. The oxygen sensor varies the voltage within a range of about 1,000 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 condition or a lean condition by decreasing or increasing the 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.
^ The system voltage is more than 9 volts.
^ The closed loop commanded air/fuel ratio is between 14.5-14.8.
^ The throttle angle is between 3-40 percent.

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

OR

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

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

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
Check for the following conditions:
^ The 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.
^ The fuel pressure-The system will go lean if the fuel pressure is too low. The PCM can compensate for some decrease. If the fuel pressure is too low, a DTC P0131 may be set.
^ The fuel injector-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.
- An incorrectly-installed or malfunctioning crankcase ventilation valve.
- Vacuum leaks at the throttle body, the Exhaust Gas Recirculation (EGR) valve, and the intake manifold mounting surfaces
^ Exhaust leaks-An exhaust leak may cause the 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
^ The Mass Air Flow (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 (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 intermittent, reviewing the Failure Records can help to determine 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 step number on the diagnostic table.
3. This DTC failing during power enrichment operation, when the Throttle Position (TP) angle 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.