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 Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Clear Memory Mode . and Inspection Mode Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Inspection Mode .
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 .

Fig 1: A/F/O2 Heater Control Circuit Low Bank 1 Sensor 1 - 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 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 and front oxygen (A/F) sensor connector.
CHECK GROUND CIRCUIT FOR ECM.
Measure the resistance of harness between ECM 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 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. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>Front Oxygen (A/F) Sensor .