P0131

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of about 450 mV between the Heated Oxygen Sensor (HO2S) signal circuits and the low circuits. When measured with a 10-megaohm digital voltmeter, this voltage 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 the injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended time, DTC P0131 will be 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, P0300, 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 14.5:1 and 14.8:1.
^ The throttle angle is between 3-40 percent.

CONDITIONS FOR SETTING THE DTC
^ The HO2S 1 signal voltage remains less than 175 mV during normal closed loop operation, OR
^ the HO2S 1 signal voltage remains less than 600 mV during power enrichment mode fuel control operation.
^ Either condition is present for up to 60 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 pigtail may be routed incorrectly and contacting the exhaust system.
^ For any faulty PCM to engine block grounds, refer to Ground Distribution Schematics.
^ The fuel 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. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ For correct fuel injector operation, refer to Fuel Injector Coil Test. Component Tests and General Diagnostics
^ Check for the following vacuum conditions:
- Any disconnected or damaged vacuum hoses-Refer to Emission Hose Routing Diagram
- An incorrectly-installed or malfunctioning crankcase ventilation valve-Refer to Crankcase Ventilation System Inspection/Diagnosis.
- Vacuum leaks at the throttle body, the Exhaust Gas Recirculation (EGR) valve, and the intake manifold mounting surfaces
^ An exhaust leak can 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 in Exhaust System.
^ Check the throttle body inlet screen for damage or for objects that may alter the airflow sample through the Mass Air Flow (MAF) sensor. If a condition is found, correct the condition as necessary. Refer to Throttle Body Air Inlet Screen Replacement.
^ Disconnect the MAF sensor. If the lean condition is corrected with the MAF sensor disconnected, replace the MAF sensor. Refer to Mass Air Flow (MAF) Sensor Replacement.
^ Check for 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, or refer to Heated Oxygen Sensor (HO2S) 2 Replacement.

TEST DESCRIPTION

Steps 1-9:

The number below refers to the step number on the diagnostic table:
3. This DTC tailing during power enrichment operation Throttle Position (TP) angle 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 procedure may help to determine how often the condition that caused the DTC to be set occurs.