P0131

DTC P0131 Heated Oxygen Sensor (HO2S 1) Circuit Low Voltage (Bank 1, Sensor 1):

Heated Oxygen Sensor (HO2S 1) Circuit:

CIRCUIT DESCRIPTION
The Vehicle Control Module-A (VCM-A) supplies a voltage of about 0.45 volt (450 mV) between terminals "BL21" and "BL7". (If measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volt) The Heated Oxygen Sensor (HO2S1) 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. DTC P0131 is a type "A" DTC.

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 determines if the Heated Oxygen Sensor (HO2S) or circuit is shorted to low by checking for a lean condition during steady throttle and power enrichment.

DTC PO131 WILL SET WHEN
^ No Throttle Position (TP) sensor DTCs.
^ No Evaporative Emissions (EVAP) System DTCs.
^ No Intake Air Temperature (IAT) sensor DTCs.
^ No Manifold Absolute Pressure (MAP) DTCs.
^ No fuel trim DTCs.
^ No Exhaust Gas Recirculation (EGR) DTCs.
^ No Engine Coolant Temperature (ECT) sensor DTCs.
^ "Closed Loop."
^ Air/fuel ratio is between 14.5 and 14.8.
^ Throttle position is between 5% and 50%.
^ Above met for 5 seconds.
^ HO2S voltage is less than 0.26V.

ACTION TAKEN (VCM-A WILL DEFAULT TO)
A current DTC P0131 set will cause the system to operate in "Open Loop." With DTC PO131 set, the Malfunction Indicator Lamp (MIL) will be turned "ON" the first test failure.

DTC P0131 WILL CLEAR WHEN
The VCM-A will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

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 indicate the exhaust oxygen content accurately.
2. Opening the heated oxygen sensor 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 1668 or a faulty VCM-A.

DIAGNOSTIC AIDS
Using the Tech 1, observe the Long Term (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 PO131 may have been set. If the conditions for DTC P0131 exist, the L.T. fuel trim values will be around 158 or greater.

Check for:
^ 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 VCM-A to engine block ground.
^ Lean injector(s) - Perform poppet nozzle test. Chart A-7 Poppet Nozzle Diagnosis
^ 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 Poppet Nozzle Diagnosis. Chart A-7 Poppet Nozzle Diagnosis
^ 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.