P0131

Diagnostic Chart:

Schematic:

CIRCUIT DESCRIPTION
The PCM supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 10 megohm digital voltmeter, this may display as low as 350 mV. The oxygen sensor varies the voltage within a range of about 1000 mV when the exhaust is rich, down through about 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 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
^ No active TP sensor, EVAP system, misfire, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, EGR Pintle Position, ECT sensor, CKP sensor, or MAF sensor DTCs present.
^ System voltage is greater than 9 volts.
^ Closed loop commanded air/fuel ratio is between 14.5 and 14.8.
^ Throttle angle is between 3% and 40%.

CONDITIONS FOR SETTING THE DTC
^ HO2S 1 signal voltage remains below 175 mV during normal closed loop operation.
OR
^ HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation.
^ Either condition is present for up to 60 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

DIAGNOSTIC AIDS
Check for the following conditions:
^ Heated oxygen sensor wiring.
The sensor pigtail may be routed incorrectly and contacting the exhaust system.
^ Poor PCM to engine block grounds.
Refer to Ground Distribution Schematics.
^ Fuel pressure.
The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0131 may be set.
Refer to Fuel System Pressure Test. Fuel System Pressure Test

^ Fuel injectors.
Refer to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Between 10-35 Degrees C (50-95 Degrees F).
Refer to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Outside 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics

^ Vacuum leaks.
Check for the following conditions:
- Disconnected or damaged vacuum hoses.
Refer to Emission Hose Routing Diagram
- Incorrectly installed or malfunctioning crankcase ventilation valve.
Refer to Positive Crankcase Ventilation Valve
- Vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
^ Exhaust leaks.
An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust Leakage (Not OBD II).
^ MAF sensor.
Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the MAF sensor.
^ Fuel contamination.
Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. Component Tests and General Diagnostics
^ If none of the above conditions are present, replace the affected HO2S.

TEST DESCRIPTION
Numbers below refer to step numbers on the Diagnostic Table:
3. This DTC failing during power enrichment operation (TP angle above 60%) may indicate a condition described in diagnostic aids. If this DTC test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
6. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.