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 DMM, this may display as low as 350 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 or lean condition by decreasing or increasing injector pulse width as necessary. If the HO2S voltage remains excessively low for an extended period of time, this Diagnostic Trouble Code (DTC) will be set.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0107, P0108, P0112; P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
^ The vehicle is in Closed Loop.
^ The air/fuel ratio is more than or equal to 14.5, but less than or equal to 14.8.
^ The Accelerator Pedal Position (APP) indicated angle is between 5-40 percent.
^ The engine run time is more than 4 minutes.
^ The Engine Coolant Temperature (ECT) more than 50°C (122°F).

CONDITIONS FOR SETTING THE DTC
The HO2S signal voltage remains less than 175 mV during normal Closed Loop operation

OR

The HO2S signal voltage remains less than 600 mV during power enrichment.

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.

TEST DESCRIPTION

Steps 1-9:

The numbers below refer to step numbers on the diagnostic table.
3. This DTC failing during power enrichment operation, with the Throttle Position (TP) angle above 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.
6. The new PCM must be programmed.