DTC P0031: A/F/O2 Heater Control Circuit Low Bank 1 Sensor 1: Notes

DTC detecting condition: 

Immediately at fault recognition

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK POWER SUPPLY TO FRONT OXYGEN (A/F) SENSOR  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connectors from front oxygen (A/F) sensor.
   3. Turn the ignition switch to ON.
   4. Measure the voltage between front oxygen (A/F) sensor connector and engine ground.

      Connector & terminal 

      (B379) No. 1 (+) - Engine ground (-):

   Is the voltage 10 V or more?

   Yes:  Go to  2

   No:  Repair the power supply line.

   NOTE: In this case, repair the following item:

   • Open circuit in harness between A/F, oxygen sensor relay and front oxygen (A/F) sensor connector
   • Poor contact of A/F, oxygen sensor relay connector
   • Malfunction of A/F, oxygen sensor relay
2. CHECK HARNESS BETWEEN ECM AND FRONT OXYGEN (A/F) SENSOR CONNECTOR  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from ECM.
   3. Measure the resistance between ECM connector and front oxygen (A/F) sensor connector.

      Connector & terminal 

      (B136) No. 5 - (B379) No. 2:

      (B136) No. 6 - (B379) No. 2:

   Is the resistance less than 1 Ω?

   Yes:  Go to  3

   No:  Repair the open circuit of harness between ECM connector and front oxygen (A/F) sensor connector.

3. CHECK GROUND CIRCUIT FOR ECM  .

   Measure the resistance of harness between ECM connector and chassis ground.

   Connector & terminal 

   (B134) No. 3 - Chassis ground:

   (B134) No. 4 - Chassis ground:

   (B134) No. 6 - Chassis ground:

   (B137) No. 1 - Chassis ground:

   (B137) No. 3 - Chassis ground:

   Is the resistance less than 5 Ω?

   Yes:  Go to  4

   No:  Repair the harness and connector.

   NOTE: In this case, repair the following item:

   • Open circuit of harness between ECM connector and engine ground
   • Poor contact of coupling connector
4. CHECK FRONT OXYGEN (A/F) SENSOR  .

   Measure the resistance between front oxygen (A/F) sensor terminals.

   Terminals 

   No. 1 - No. 2:

   Is the resistance 2 - 3 Ω?

   Yes:  Repair the poor contact of ECM connector.

   No:  Replace the front oxygen (A/F) sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Front Oxygen (A/F) Sensor .