Code No. P0031: Oxygen Sensor (front) Heater Circuit Low Input <Except vehicles for Russia>


OPERATION



  • Power is supplied to the heater power terminal (terminal No. 3) of the oxygen sensor (front) connector from the engine control relay (terminal No. 2).
  • The heater (terminal No. 4) of the oxygen sensor (front) connector is controlled by the power transistor in the engine-ECU (terminal No. 34).

FUNCTION



  • The power supply to the oxygen sensor (front) heater is controlled by the ON/OFF control of the power transistor in the engine-ECU.
  • Heating the oxygen sensor (front) heater enables the oxygen sensor (front) to provide good response even when the exhaust emission temperature is low.

TROUBLE JUDGMENT


Check Conditions
  • More than 2 seconds have passed since the engine starting sequence was completed.
  • While the oxygen sensor (front) heater is on.
  • Battery positive voltage is more than 11 V and is less than 16.5 V.
Judgement Criterion
  • The oxygen sensor (front) heater current is less than 0.17 A for 2 seconds.
Check Conditions
  • More than 2 seconds have passed since the engine starting sequence was completed.
  • While the oxygen sensor (front) heater is off.
  • Battery positive voltage is more than 11 V and is less than 16.5 V.
Judgement Criterion
  • The oxygen sensor (front) heater voltage is less than 2.0 V for 2 seconds.

FAIL-SAFE AND BACKUP FUNCTION



  • None

PROBABLE CAUSES



  • Failed oxygen sensor (front) heater
  • Open/short circuit or harness damage in oxygen sensor (front) heater circuit or loose connector contact
  • Failed engine-ECU

DIAGNOSIS PROCEDURE

STEP 1. Connector check: B-32 oxygen sensor (front) connector

Q. Is the check result normal?
Go to Step 2.
Repair or replace the connector.

STEP 2. Perform resistance measurement at B-32 oxygen sensor (front) connector.

  • Disconnect connector, and measure at sensor side.
  • Resistance between terminal No. 3 and No. 4.
OK: 4.5 - 8.0 Ω (at 20°C)

Q. Is the check result normal?
Go to Step 3.
Replace the oxygen sensor (front).

STEP 3. Perform voltage measurement at B-32 oxygen sensor (front) connector.

  • Disconnect connector, and measure at harness side.
  • Ignition switch: ON
  • Voltage between terminal No. 3 and earth.
OK: System voltage

Q. Is the check result normal?
Go to Step 5.
Go to Step 4.

STEP 4. Connector check: A-37X engine control relay connector

Q. Is the check result normal?

    Check intermediate connector A-17, and repair if necessary. If intermediate connector is normal, check and repair harness between B-32 (terminal No. 3) oxygen sensor (front) connector and A-37X (terminal No. 2) engine control relay connector.

    • Check power supply line for open/short circuit.
Repair or replace the connector.

STEP 5. Connector check: B-26 engine-ECU connector

Q. Is the check result normal?
Go to Step 6.
Repair or replace the connector.

STEP 6. Perform voltage measurement at B-26 engine-ECU connector.

  • Disconnect connector, and measure at harness side.
  • Ignition switch: ON
  • Voltage between terminal No. 34 and earth.
OK: System voltage

Q. Is the check result normal?
Go to Step 7.

    Check and repair harness between B-32 (terminal No. 4) oxygen sensor (front) connector and B-26 (terminal No. 34) engine-ECU connector.

    • Check earthing line for open/short circuit.

STEP 7. Check harness between B-32 (terminal No. 4) oxygen sensor (front) connector and B-26 (terminal No. 34) engine-ECU connector.

  • Check earthing line for damage.

Q. Is the check result normal?
Go to Step 8.
Repair the damaged harness wire.

STEP 8. Connector check: A-37X engine control relay connector

Q. Is the check result normal?
Go to Step 9.
Repair or replace the connector.

STEP 9. Check harness between B-32 (terminal No. 3) oxygen sensor (front) connector and A-37X (terminal No. 2) engine control relay connector.
note Before checking harness, check intermediate connector A-17, and repair if necessary.

  • Check power supply line for damage.

Q. Is the check result normal?
Go to Step 10.
Repair the damaged harness wire.

STEP 10. M.U.T.-III diagnosis code

  • Reconfirmation of diagnosis code.

Q. Is diagnosis code set?
Replace the engine-ECU.
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).