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 Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Clear Memory Mode>OPERATION , and inspection mode Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Inspection Mode>PROCEDURE .
Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram:

Engine Electrical System Ref. 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. 2 (+) - 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 of harness between M/B main relay connector and front oxygen (A/F) sensor connector
- Poor contact of M/B main relay connector
- Poor contact of coupling connector
- Defective M/B main 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
  
  (E158) No. 5 - (E24) No. 1:
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 chassis ground.

Connector & terminal

(E158) No. 61 - Chassis ground:

(E158) No. 62 - Chassis ground:

(E158) No. 63 - Chassis ground:

(E158) No. 64 - Chassis ground:

(E158) No. 65 - Chassis ground:

Is the resistance less than 5 Ω?

Yes: Go to 4

No: Repair the open circuit in harness between ECM connector and engine ground.
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) (H4DO)>FRONT OXYGEN (A/F) SENSOR