P0131

Code P0131, Heated Oxygen Sensor (HO2S) Circuit (Low Signal Voltage):

Heated Oxygen Sensor (HO2S) Circuit:

CIRCUIT DESCRIPTION
The PCM supplies a voltage of about 0.45 volt (450 mV) between terminals "BF9" and "BF10". (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.


DTC P0131 WILL SET WHEN:
^ The HO2S signal voltage on CKT 412 remains below 97 mV for at least 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 MIL (Service Engine Soon) will be illuminated.

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 mis-positioned and contacting the exhaust manifold. Check for intermittent ground in wire between connector and sensor, or poor PCM to engine block ground.

^ MAF 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 "CHART C-2A" under "DIAGNOSTIC CHARTS/C-CHARTS" Chart C-2A Injector Coil Balance Test Procedure

^ 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 "Fuel System Diagnosis," CHART A-7. Chart A-7 Fuel System Pressure Test

^ Exhaust leaks - If there is an exhaust leak, the engine can cause outside air to be pulled into the exhaust and past the sensor, causing a low signal voltage and false lean indication.

^ 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.