P0131

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a bias voltage of approximately 450 mV between the Heated Oxygen Sensor (HO2S) signal and the low circuits. When measured with a 10-megaohm DMM, this may display as low as 350 mV. The oxygen sensor varies the voltage from approximately 100 mV when the exhaust is rich, to approximately 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 set.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P010?, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-206, P0410, P0440, P0442, P0443, P0446, P0449, and P1441 are not set.
^ The system voltage is more than 9 volts.
^ The Closed Loop commanded air-to-fuel ratio is between 12-16.5.
^ The throttle angle is between 3-35 percent.

CONDITIONS FOR SETTING THE DTC
^ The HO2S 1 signal voltage remains below 175 mV during normal Closed Loop operation.

OR

^ The HO2S 1 signal voltage remains below 600 mV during Power Enrichment Mode fuel control operation.
^ Either of the above conditions is present for up to 50 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.
^ Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

Steps 1-5:

Steps 6-11:

The number below refers to the step number on the diagnostic table.
3. In order to determine if the engine is lean during power enrichment, operate the vehicle under higher throttle, or higher acceleration, conditions. Monitor the scan tool HO2S voltage and the power enrichment parameter at the higher acceleration. A lean condition will cause the HO2S voltage to be less than 600 mV during power enrichment.