P0131

Circuit Description
The PCM supplies a bias voltage of about 450mV between the HO2S signal and low circuits. When measured with a 10 megohm digital voltmeter, this may display as low as 350mV. The oxygen sensor varies the voltage within a range of about 1000mV when the exhaust is rich, down through about 100mV 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 DTC 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 175mV during normal closed loop operation.
or
^ HO2S 1 signal voltage remains below 6OOmV during power enrichment mode fuel control operation.
^ Either condition 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 tailed.
^ 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 a scan tool.

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.
^ 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 (VIN 1). Fuel System Pressure Test
^ Fuel injector(s).
Refer to Fuel Inj Coil Test - ECT Between 10-35 Degrees C or Fuel Inj Coil Test - ECT Outside 10-35 Degrees C. 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 Crankcase Ventilation System Inspection.
- 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).
^ Throttle Body Inlet Screen.
Check for damage or objects that may alter the airflow sample through the MAF sensor. If a problem is found, correct as necessary. Refer to Throttle Body Air Inlet Screen Replacement
^ MAF sensor Disconnect the MAF sensor. If the lean condition is corrected with the MAF sensor disconnected, replace MAF sensor. Refer to MAF Sensor Replacement.
^ 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.
^ If none of the above conditions are present, replace the affected HO2S. Refer to HO2S Replacement - Sensor 1.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
3. DTC P0131 failing during power enrichment operation (TP angle above 60%) may indicate a condition described in Diagnostic Aids. If the DTC P0131 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
6. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.