P0131

DTC P0131:

Heated Oxygen Sensor Wiring Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) supplies a voltage of about 0.45 volt (450 mV) between terminals "BD2" and "BD3". (If measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volt.) The Heated Oxygen Sensor (HO2S) varies the voltage within a range of about 1 volt (1000 mV) if the exhaust is rich, down to about 0.10 volt (100 mV) if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below about 360°C (600°F). DTC P0131, P0132 or an open sensor circuit or cold sensor causes "Open Loop" operation.

DIAGNOSTIC TROUBLE CODE (DTC) PO131 WILL SET WHEN
^ The HO2S signal voltage on CKT 412 remains below 248 mV for at least 4.2 minutes.
^ The system is operating in "Closed Loop."

ACTION TAKEN (PCM WILL DEFAULT TO)
A current DTC P0131 set will cause the system to operate in "Open Loop." With DTC P0131 set, the Malfunction Indicator Lamp (MIL) (Service Engine Soon) will be illuminated.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Running the engine at 1200 RPM keeps the oxygen sensor hot to ensure the oxygen sensor remains active and can read the exhaust oxygen content accurately.
2. Opening the HO2S signal circuit and grounding the HO2S low circuit should result in a displayed voltage between 350 and 550 mV. If the display is still fixed below 350 mV, the fault is a short to ground in CKT 412 or a faulty PCM.

DIAGNOSTIC AIDS
Using the Tech 1, observe the L.T. fuel trim values at different RPM and air flow conditions. The Tech 1 also displays the fuel trim cells, so the L.T. fuel trim values can be checked in each of the cells to determine when DTC P0131 may have been set. If the conditions for DTC P0131 exist, the L.T. fuel trim values will be around 158 or greater.

Heated Oxygen Sensor Wire
Sensor pigtail may be mispositioned and contacting the exhaust manifold.
Check for intermittent ground in wire between connector and sensor.
Poor PCM to engine block ground.

Mass Air Flow Sensor
A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause the system to go lean. Disconnect the MAF sensor. If the lean condition is gone, replace the MAF sensor.

Lean Injector(s)
Perform injector balance test. See CHART C-2A. Chart C-2A Fuel Injector Balance Test

Fuel Contamination
Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC P0131.

Fuel Pressure
System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to CHART A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

Exhaust Leaks
If there is an exhaust leak, the engine can cause outside air to be pulled into the exhaust and past the sensor.
Vacuum or crankcase leaks can cause a lean condition and/or possibly a high idle.

If the above are OK, it is a faulty heated oxygen sensor.
Faulty HO2S heater or heater circuit - See Diagnostic Aids DTC P0134. P0134