P0131

Schematic:

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV. The Powertrain Control Module (PCM) monitors this voltage and determines by the exhaust gas oxygen content whether the air/fuel mixture is rich or lean. When the voltage input at the PCM is approximately 100 mV, the HO2S is indicating a lean air/fuel mixture. When the voltage input is approximately 900 mV, the HO2S is indicating rich air/fuel mixture. The PCM constantly monitors the HO2S signal during closed-loop operation. The PCM compensates for a rich condition or for a lean condition by changing the fuel-injector pulse width. A DTC P0131 will be set if the HO2S 1 voltage remains excessively low for an extended period of time.

CONDITIONS FOR RUNNING THE DTC
^ The barometric pressure is more than 75 kPa.
^ The intake air temperature is between -10°C and +70°C (14°F - 158°F).
^ The fuel tank level is more than 25 percent.
^ The engine coolant temperature is above 80°C (176°F).
^ The vehicle speed exceeds 50 km/h (30 mph) for 2 minutes, after which the vehicle idles for 2 minutes.
^ The engine is running at a steady speed.

CONDITIONS FOR SETTING THE DTC
The maximum HO2S voltage is less than 300 mV.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
^ The PCM records the operating conditions at the time that the diagnostic fails. This information is stored in the Freeze Frame butter.

CONDITIONS FOR CLEARING THE DTC
^ The MIL turns OFF after 3 consecutive trips during which the diagnostic runs and does not detect a fault.
^ A History DTC clears after 40 consecutive warm-up cycles without a fault.
^ Use the Clear DTC Information function, or disconnect the PCM battery feed in order to clear the DTC.

DIAGNOSTIC AIDS
DTC P0131 normally sets during city driving. Avoid operating the vehicle on a highway when validating DTC P0131.

Clear the DTCs. Road-test the vehicle while monitoring the HO2S test in the MIL/System Status selection of System Information. When the scan tool indicates YES, check for DTC P0131 in the Last Test Failed. If the scan tool does not indicate DTC P0131, the DTC is intermittent, and the HO2S system diagnostic is indicating OK at this time.

Check for any of the following conditions:
^ Any misrouted HO2S wiring. The sensor pigtail may be contacting the exhaust system and causing a short to ground.
^ Check for good PCM-to-engine electrical grounds.
^ Incorrect fuel pressure. The fuel system can go lean if the fuel pressure is too low. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics

An incorrectly calibrated fuel injector
^ Any vacuum leaks. Check for disconnected vacuum hoses and for damaged vacuum hoses. Check for vacuum leaks at the intake manifold, at the throttle body, and at the crankcase ventilation system. Any exhaust leaks near the HO2S. An exhaust leak may cause outside air to be pulled into the exhaust gas stream and past the HO2S.
^ The outside air may cause a DTC P0131 to set. A shorted HO2S. An HO2S shorted to ground measures less than 0.1 volts on the scan tool. Water contamination of the HO2S can cause a low HO2S voltage to be indicated. A DTC P0131 that occurs when driving through standing water or when operating the vehicle in wet driving conditions can indicate water intrusion. Replace the HO2S if the sensor is affected by moisture. A contaminated HO2S-Silicon contamination of the HO2S can cause low signal voltage to be indicated.
^ A powdery white deposit on the portion of the HO2S that is exposed to the exhaust stream indicates silicon contamination. Determine the cause of the condition, and correct the condition before replacing the HO2S. Check for fuel contamination.
^ Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to A faulty Alcohol/Contaminants-in-Fuel Diagnosis electrical connection to the PCM. Component Tests and General Diagnostics
^ An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit.

Inspect the wiring harness and the components for any of the following conditions: Backed out terminals
^ Incorrect mating of terminals
^ Broken electrical connector locks
^ Incorrectly formed terminals or damaged terminals
^ Faulty terminal-to-wire connections
^ Physical damage to the wiring harness
^ A broken wire inside the insulation
^ Corrosion of electrical connections, of splices, or of terminals
^ If DTC P0131 cannot be duplicated, the Freeze Frame data can be useful in determining the operating conditions when the DTC first set.