Code No. P0125: Feedback System Monitor


OPERATION



FUNCTION



TROUBLE JUDGMENT


Check Conditions
Judgment Criterion

PROBABLE CAUSES



note When the oxygen sensor (front) begins to deteriorate, the oxygen sensor output voltage will deviate from the voltage when the sensor was new (normally 0.5 volt at stoichiometric ratio). This deviation will be corrected by the oxygen sensor (rear).If the oxygen sensor (rear) responds poorly because it has deteriorated, it will improperly correct the oxygen sensor (front). Thus, even when closed loop control is being effected, the fluctuation of the oxygen sensor (front) output voltage decreases, without intersecting with 0.5 volt. As a result, there is a possibility of diagnosis code P0125 becoming registered.

DIAGNOSIS PROCEDURE

STEP 1. M.U.T.-III diagnosis code

Q. Is any other diagnosis code than P0125 output?
Inspection chart for diagnosis code (Refer to ).
Go to Step 2 .

STEP 2. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item No. 8: Engine coolant temperature sensor
  2. Item No. 9: Oxygen sensor (front)
  3. Item No. 10: Oxygen sensor (rear)
  4. Item No. 80: Air flow sensor

Q. Are the check results normal?
Go to Step 3 .
Perform the diagnosis code classified check procedure for the sensor that has shown an abnormal data valve (Refer to Inspection Chart for Diagnosis Code ).

STEP 3. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item No. 79: Barometric pressure sensor

Q. Is the check result normal?
Go to Step 4 .
Replace the engine-ECU then perform the variant coding (Refer to GROUP 00 - Precautions Before Service - How to Perform Variant Coding ).

STEP 4. Check for intake of air from gaps in gasket, seals, etc.

Q. Is the check result normal?
Go to Step 5 .
Repair for intake of air from gaps in gasket, seals, etc.

STEP 5. Check for leakage of exhaust emission from exhaust manifold.

Q. Is the check result normal?
Go to Step 6 .
Repair for leakage of exhaust emission from exhaust manifold.

STEP 6. Check throttle body (throttle valve portion) for contamination.


Q. Is the check result normal?
Go to Step 7 .
Clean throttle body (throttle valve portion) (Refer to ).

STEP 7. Connector check: A-145 oxygen sensor (front) connector, A-35 engine-ECU connector.


Q. Is the check result normal?

Go to Step 8 .

Repair or replace the connector.

STEP 8. Check A-145 oxygen sensor (front) itself.

  • Check oxygen sensor (front) itself (Refer to ).

Q. Is the check result normal?
Go to Step 9 .
Replace the oxygen sensor (front).

STEP 9. Check harness between A-145 (terminal No. 1) oxygen sensor (front) connector and A-35 (terminal No. 97) engine-ECU connector.



  • Check output line for damage.

Q. Is the check result normal?

Go to Step 10 .

Repair the damaged harness wire.

STEP 10. Connector check: Injector connector


  1. A-101 (No.1 injector connector)
  2. A-102 (No.2 injector connector)
  3. A-123 (No.3 injector connector)
  4. A-121 (No.4 injector connector)

Q. Is the check result normal?

Go to Step 11 .

Repair or replace the connector.

STEP 11. Check injector itself.

  • Check Injector itself (Refer to ).

Q. Is the check result normal?
Go to Step 12 .
Replace the injector.

STEP 12. Connector check: B-206 engine control relay connector and A-34 engine-ECU connector.


Q. Is the check result normal?

Go to Step 13 .

Repair or replace the connector.

STEP 13. Check harness between B-206 engine control relay connector and injector connector.



  1. Check harness between B-206 (terminal No. 4) engine control relay connector and A-101 (terminal No. 1) No. 1 injector connector.
  2. Check harness between B-206 (terminal No. 4) engine control relay connector and A-102 (terminal No. 1) No. 2 injector connector.
  3. Check harness between B-206 (terminal No. 4) engine control relay connector and A-123 (terminal No. 1) No. 3 injector connector.
  4. Check harness between B-206 (terminal No. 4) engine control relay connector and A-121 (terminal No. 1) No. 4 injector connector.
    note Before checking harness, check intermediate connectors B-212 and A-36, and repair if necessary.

    • Check power supply line for damage.

Q. Are the check results normal?

Go to Step 14 .

Repair the damaged harness wire.

STEP 14. Check harness between injector connector and A-34 engine-ECU connector.



  1. Check harness between A-101 (terminal No. 2) No. 1 injector connector and A-34 (terminal No. 8) engine-ECU connector.
  2. Check harness between A-102 (terminal No. 2) No. 2 injector connector and A-34 (terminal No. 9) engine-ECU connector.
  3. Check harness between A-123 (terminal No. 2) No. 3 injector connector and A-34 (terminal No. 23) engine-ECU connector.
  4. Check harness between A-121 (terminal No. 2) No. 4 injector connector and A-34 (terminal No. 24) engine-ECU connector.

  • Check earthing line for damage.

Q. Are the check results normal?

Go to Step 15 .

Repair the damaged harness wire.

STEP 15. Fuel pressure measurement.

  • Fuel pressure measurement (Refer to Fuel Pressure Test ).

Q. Is the check result normal?
Replace the engine-ECU then perform the variant coding (Refer to GROUP 00 - Precautions Before Service - How to Perform Variant Coding ).
Repair the fuel pressure.