P0131 or P0151 (Federal W/RPO L35)

CIRCUIT DESCRIPTION
The heated oxygen sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater that is necessary in order to quickly warm the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches operating temperature, the control module monitors the HO2S bias, or reference, voltage. The control module also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or lean out the mixture, when the HO2S detects a rich exhaust condition.

This DTC is designed to detect an HO2S voltage that remains low for more than a specified number of seconds during the test conditions.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0401, P0405, P0440, P0442, P0452, P0453, P1404, or P1441 are not set.
^ The ignition 1 signal is between 9-18 volts.
^ The fuel tank level remaining is more than 10 percent.
^ The loop status is closed.
^ Intrusive tests are not in progress.
^ Scan tool output controls are not active.

Lean Test Enable
^ The air fuel ratio is between 14.5:1-14.7:1.
^ The throttle position (TP) is between 3-70 percent.

Power Enrichment Test Enable
^ Power Enrichment is active.
^ The above conditions are met for 2 seconds.

CONDITIONS FOR SETTING THE DTC

Lean Test
The HO2S voltage is lees than 87 mV for 180 seconds.

Power Enrichment Test
The HO2S voltage is less than 360 mV for 10 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.
^ The control module commands Open Loop.

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

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

^ Inspect the heated oxygen sensor (HO2S) wire - The sensor pigtail may be in contact with the exhaust manifold or with the exhaust system. An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provide the supply of oxygen. Inspect the HO2S wires and inspect the connections for breaks or for contamination - Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagnostic Aids.
^ Check for intermittent ground in the signal wire between the connector and the sensor - Refer to Testing for Intermittent and Poor Connections in Diagnostic Aids.
^ Inspect for lean injectors. Perform the Injector Balance Test - Refer to Fuel Injector Balance Test with Tech 2 or Fuel Injector Balance Test with Special Tool. Component Tests and General Diagnostics
^ Inspect for fuel contamination-Water near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC. Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
^ Inspect the fuel pressure - The system will be lean if fuel the pressure is low. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Inspect for exhaust leaks - An exhaust leak near the HO2S can cause a lean condition. Refer to Exhaust Leakage in Engine Exhaust.
^ Inspect for vacuum or crankcase leaks that can cause a lean condition.
^ If the above components are OK, the HO2S may be at fault.

For an intermittent condition, refer to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures

TEST DESCRIPTION

DTC P0131 Or P0151 (Federal W/RPO L35) - Steps 1-5:

DTC P0131 Or P0151 (Federal W/RPO L35) - Steps 6-13:

The numbers below refer to the step numbers on the diagnostic table.
2. If the voltage is below the specified value the condition is present.
4. Jumpering the HO2S low signal circuit to ground is necessary for the PCM to correctly measure the voltage on the HO2S high signal circuit. If the HO2S voltage is within the specified range, the PCM and HO2S high signal circuit are OK.

For a description of the RPO Code(s) shown in this article or any of the images therein, refer to the RPO Code List found at Vehicle/Application ID".