P0131

Diagnostic Chart:

Wiring Diagram:

Circuit Description

The Heated Oxygen Sensor (HO2S) produces a voltage that varies between 1 00mV and 900mV under normal operating conditions. The Powertrain Control Module (PCM) monitors this voltage and determines if the exhaust is lean or rich. If the voltage input at the PCM is about 100 mV the exhaust is lean, and if the voltage input is about 900 mV the exhaust is rich. The PCM constantly monitors the HO2S signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing fuel injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended period of time, DTC P0131 will be set.

Conditions for Running the DTC

- Vehicle speed above 32 km/h (20 mph).

- Engine rpm greater than 2000 PPM.

- In closed loop for 20 seconds.

- Barometric pressure greater than 75 kPa.

Conditions for Setting the DTC

- Maximum voltage average is less than 500 mV and minimum voltage average is less than 7O mV.

- Pull-up voltage greater than 4.5 V for 2 seconds.

Action Taken When the DTC Sets

- The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.

- The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC

- The MIL will turn off after three consecutively passing cycles without a fault present.

- A History DTC will clear after 40 consecutive warm-up cycles without a fault.

- DTC(s) can be cleared by using a scan tool or by disconnecting the PCM battery feed.

Diagnostic Aids

Check for any of the following conditions:

- If any DTCs other than a Fuel Trim (P0171, P0172), Misfire (P0300 to P0304), EGR (P0400) and HO2S 1 (P0131 to P0134) are set, diagnosis those DTCs first.

- Misrouted heated oxygen sensor wiring. The sensor pigtail may be mis-positioned and contacting the exhaust system.

Poor PCM to engine grounds.

- Fuel pressure. The fuel system can go lean if the fuel pressure is too low and a DTC P0131 may be set. Refer to Fuel System Diagnosis. Table A-7B Fuel System Diagnosis (Fuel Pressure Check)

- Lean fuel injector(s)

- Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.

- Exhaust leaks. An exhaust leak may cause outside air to be pulled into the exhaust gas stream and flowed past the HO2S, causing a DTC P0131 to set.

Check for exhaust leaks near the HO2S.

- A shorted Heated Oxygen Sensor (HO2S). If the HO2S is shorted to ground the HO2S reading displayed on the scan tool will be under 0.1 volts. Water contamination of the HO2S can cause a low HO2S voltage to be indicated. Note if the DTC P0131 occurs when driving through standing water or when operating the vehicle in wet driving conditions. Replace the HO2S if it is adversely affected by moisture

- A contaminated HO2S. Silicon contamination of the HO2S can cause a low HO2S voltage to be indicated. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is found, determine the cause and correct the condition before replacing the HO2S. Refer to Heated Oxygen Sensor 1 (HO2S 1).

- Fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. Too much alcohol in the fuel can also cause a lean exhaust indication. Refer to Fuel for procedures to check for fuel contamination. Fuel

If the DTC P0131 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

If any wiring repairs need to be made, Refer to Repair Procedures in Diagrams/Electrical.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. Determine if fault is present, vehicle must be at operating temperature.

3. Review Freeze Frame data to determine when the DTC set. Always record this information.