Code No. P0031: Oxygen Sensor (Front) Heater Circuit Low Input












OPERATION



  • Power is supplied to the heater power terminal (terminal No. 1) of the oxygen sensor (front) connector from the engine control relay (terminal No. 2).
  • The heater (terminal No. 2) 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 heater (front) 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 11 - 16.5 V.
Judgement Criterion
  • The oxygen sensor (front) heater current has continued to be lower 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 11 - 16.5 V.
Judgement Criterion
  • The oxygen sensor (front) heater voltage has continued to be lower than 2.0 V for 2 seconds.

PROBABLE CAUSES



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

DIAGNOSIS PROCEDURE


STEP 1. Connector check: C-134 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 C-134 oxygen sensor (front) connector.



  • Disconnect connector, and measure at sensor side.
  • Resistance between terminal No. 1 and No. 2.
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 C-134 oxygen sensor (front) connector.



  • Disconnect connector and measure at harness side.
  • Ignition switch: ON
  • Voltage between terminal No. 1 and earth.
OK: System voltage

Q. Is the check result normal?

Go to Step 5 .
Go to Step 4 .

STEP 4. Connector check: A-33X engine control relay connector


Q. Is the check result normal?


    Check and repair harness between C-134 (terminal No. 1) oxygen sensor (front) connector and A-33X (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-29 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-29 engine-ECU connector.



  • Disconnect engine-ECU harness, and connect special tool power plant ECU check harness (MB992110) to measure at connector for inspection.
  • Ignition switch: ON
  • Voltage between terminal No. 34 and earth.
OK: System voltage

Q. Is the check result normal?

Go to Step 7 .

    Check intermediate connector A-13, and repair if necessary. If intermediate connector is normal, check and repair harness between C-134 (terminal No. 2) oxygen sensor (front) connector and B-29 (terminal No. 34) engine-ECU connector.

    • Check earthing line for open/short circuit.

STEP 7. Check harness between C-134 (terminal No. 2) oxygen sensor (front) connector and B-29 (terminal No. 34) engine-ECU connector.


note Before checking harness, check intermediate connector A-13, and repair if necessary.

  • Check earthing line for damage.

Q. Is the check result normal?

Go to Step 8 .
Repair the damaged harness wire.

STEP 8. Check harness between C-134 (terminal No. 1) oxygen sensor (front) connector and A-33X (terminal No. 2) engine control relay connector.



  • Check Power supply line for damage.

Q. Is the check result normal?

Go to Step 9 .
Repair the damaged harness wire.

STEP 9. 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 ).