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

DTC detecting condition:

Immediately at fault recognition

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>CLEAR MEMORY MODE>OPERATION , and inspection mode Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>INSPECTION MODE>PROCEDURE ..
Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram:

Engine Electrical System Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

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

(E24) No. 3 (+) - 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
- Poor contact of coupling connector
- Malfunction of A/F, oxygen sensor relay
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

(E24) No. 2 - (E159) No. 9:

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.
CHECK GROUND CIRCUIT FOR ECM.
Measure the resistance of harness between ECM connector and engine ground.

Connector & terminal

(B134) No. 35 - Engine ground:

(B134) No. 47 - Engine ground:

(E159) No. 10 - Engine ground:

(E159) No. 17 - Engine ground:

(E159) No. 18 - Engine ground:

(E159) No. 25 - Engine ground:

(E159) No. 26 - Engine 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
CHECK FRONT OXYGEN (A/F) SENSOR.
Measure the resistance between front oxygen (A/F) sensor terminals.

Terminals

No. 2 - No. 3:

Is the resistance 2 - 3 Ω?

Yes: Repair the poor contact of ECM connector.

No: Replace the front oxygen (A/F) sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>FRONT OXYGEN (A/F) SENSOR .