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 .

Courtesy of SUBARU OF AMERICA, INC.

CAUTION: Use the check board when measuring the ECM terminal voltage and resistance. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>GENERAL DESCRIPTION>PREPARATION TOOL>HOW TO USE CHECK BOARD .

NOTE: After the faulty parts are repaired or replaced, perform the final check in Basic Diagnostic Procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

CHECK POWER SUPPLY OF FRONT OXYGEN (A/F) SENSOR .
1. Turn the ignition switch to OFF.
2. Disconnect the connector from the 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

(B94) No. 2 (+) - Engine ground (-):

Is the voltage 10 V or more?

Yes : Go to 2.

No : Repair the following item.
- Open circuit in harness between M/B and front oxygen (A/F) sensor connector
- Blown out of fuse
CHECK HARNESS BETWEEN ECM AND FRONT OXYGEN (A/F) SENSOR CONNECTOR (OPEN) .
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

(E53) No. 88 - (B94) No. 1:

Is the resistance less than 1 Ω?

Yes : Go to 3.

No : Repair the following item.
- Open circuit in harness between ECM connector and front oxygen (A/F) sensor connector
- Poor contact of coupling connector
CHECK GROUND CIRCUIT FOR ECM (OPEN CIRCUIT) .
Measure the resistance of harness between ECM connector and engine ground.

Connector & terminal

(B62) No. 69 - Engine ground:

(B62) No. 70 - Engine ground:

(B62) No. 71 - Engine ground:

(B62) No. 72 - Engine ground:

Is the resistance less than 5 Ω?

Yes : Go to 4.

No : 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 .
Check the front oxygen (A/F) sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FRONT OXYGEN (A/F) SENSOR .

Is the check result OK?

Yes : Repair the poor contact of ECM connector.

No : Replace the front oxygen (A/F) sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FRONT OXYGEN (A/F) SENSOR .