P0131

CIRCUIT DESCRIPTION
The heated oxygen sensor (HO2S) is a sensor used to detect the oxygen content in the exhaust. The powertrain control module (PCM) supplies the sensor with a signal circuit and a ground circuit. The PCM supplies a bias voltage between the circuits. The sensor varies the voltage based on the oxygen content in the exhaust. When the exhaust stream is lean (more oxygen), the sensor produces a low voltage signal. When the exhaust stream is rich (less oxygen), the sensor produces a high voltage signal. The PCM monitors the HO2S signal during Closed Loop. To correct for rich or lean conditions, the PCM adjusts the injector pulse width. This DTC indicates that the HO2S voltage remained too low for a calibrated period of time.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0201-P0208, P0410, P0412, P0418, P0419, P0440, P0442, P0443, P0446, P0449, P1415, P1416, or P1441 are not set.
^ The throttle position (TP) angle is between 3-25 percent.
^ The system voltage is between 9-18 volts.
^ The Closed Loop fuel control is enabled.
^ The air/fuel ratio is between 14.5-14.8.
^ The exhaust gas recirculation (EGR) flow diagnostic test is not active.
^ No injectors are disabled.
^ The secondary air injection (AIR) diagnostic test is not active.
^ The catalyst diagnostic test is not active.
^ The conditions above are met for 3 seconds.

CONDITIONS FOR SETTING THE DTC
^ The HO2S signal voltage remains below 75 mV (0.075 V) in Closed Loop mode.

or

^ The HO2S signal voltage remains below 575 mV (0.575 V) during power enrichment (PE) mode.
^ The above conditions are present for more than 5 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.

DIAGNOSTIC AIDS

NOTE: Refer to Heated Oxygen Sensor (HO2S) Wire Repair Notice in Service Precautions.

Check for the following conditions:
^ The HO2S wire routing-The sensor wiring may be routed incorrectly and contacting the exhaust system.
^ The fuel pressure-Fuel pressure that is too low will cause a lean condition. The PCM will compensate for some decrease. However, if fuel pressure is low enough, the DTC may set.
^ The fuel injectors-If the fuel injectors deliver improper fuel volume at the specified pressure, the DTC may set. Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2. Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Exhaust leaks-Exhaust leaks may cause outside air to be drawn into the exhaust gas stream, and pass the HO2S sensor. This causes the HO2S signal to indicate a false lean condition.
^ Secondary air injection (AIR) system leaks- Check the AIR system hoses, the check valves, and the piping for leaks, for poor connections or for signs of damage.
^ Vacuum leaks
Check for the following:
- Disconnected or damaged vacuum hoses
- Vacuum leaks at the throttle body, the exhaust gas recirculation (EGR) valve, the evaporative emission (EVAP) purge solenoid, and the intake manifold
^ Fuel contamination-Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust. Excessive alcohol in the fuel can cause a lean condition.
^ Check with the customer to verify the conditions during which the DTC set. Check to see if the vehicle ran out of fuel.

If none of the above conditions are present, replace the HO2S sensor.

Review the front and rear long term fuel trim values on the scan tool Failure Record data. If both banks were lean, check for a condition that would cause a lean engine operation. If only the rear bank was lean, the lean condition is isolated to that bank, such as an intermittent short to ground on the signal circuit.

If the condition is not present, check with the customer about the type of fuel used, for example over 10 percent ethanol fuel. Recommend using fuels of 10 percent or less ethanol.

If the condition is intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-8:

The numbers below refer to the step numbers on the diagnostic table.
2. This step verifies whether the HO2S sensor voltage is switching from below 325 mV to above 625 mV, or remains below 300 mV.
4. This step verifies the bias voltage from the PCM and verifies the condition of the circuit wiring. With the sensor disconnected, the scan tool should display a bias voltage near 450 mV.