Code No. P0131: Oxygen Sensor (Front) Circuit Low Voltage








OPERATION



  • The sensor signal is inputted to the engine-ECU (terminal No. 38) from the oxygen sensor (front) output terminal (terminal No. 3).
  • The oxygen sensor (front) (terminal No. 4) is earthed with engine-ECU (terminal No. 39).

FUNCTION



  • The oxygen sensor (front) converts the concentration of oxygen in the exhaust emission into a voltage and inputs the signal to the engine-ECU.
  • When the air-fuel ratio is richer than the theoretical air-fuel ratio, the oxygen sensor (front) outputs a voltage of about 1 V. When it is leaner than the theoretical air-fuel ratio, it outputs a voltage of about 0 V.
  • In response to the signal, the engine-ECU controls the fuel injection amount so that the air-fuel ratio can be equivalent to the theoretical air-fuel ratio.

TROUBLE JUDGMENT


Check Conditions
  • Oxygen sensor offset voltage is 0.4 - 0.6V.
  • Battery positive voltage is 11 - 16.5 V.
  • Typically 60 seconds have passed since the engine starting sequence was completed.
Judgement Criterion
  • Oxygen sensor (front) output voltage is lower than 0.2 V for 2 seconds.

PROBABLE CAUSES



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

DIAGNOSIS PROCEDURE

STEP 1. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item AC: Oxygen sensor (front)

Q. Is the check result normal?
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
Go to Step 2 .

STEP 2. Connector check: C-134 oxygen sensor (front) connector

Q. Is the check result normal?
Go to Step 3 .
Repair or replace the connector.

STEP 3. Perform resistance measurement at C-134 oxygen sensor (front) connector.

  • Disconnect connector, and measure at harness side.
  • Resistance between terminal No. 4 and earth.
OK: Continuity (2 Ω or less)

Q. Is the check result normal?
Go to Step 7 .
Go to Step 4 .

STEP 4. Connector check: B-29 engine-ECU connector

Q. Is the check result normal?
Go to Step 5 .
Repair or replace the connector.

STEP 5. Check harness between C-134 (terminal No. 4) oxygen sensor (front) connector and B-29 (terminal No. 39) 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 6 .
Repair the damaged harness wire.

STEP 6. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item AC: Oxygen sensor (front)

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

STEP 7. Perform voltage measurement at C-134 oxygen sensor (front) connector.

  • Use special tool test harness (MB991658) to connect connector, and measure at pick-up harness.
  • Engine: After warm-up
  • Transmission: Neutral <M/T> or P range <CVT>
  • Voltage between terminal No. 3 and earth.
OK:
    When the engine is 2,500 r/min, the output voltage should repeat 0.5 to 1.4 V alternately.

Q. Is the check result normal?
Go to Step 10 .
Go to Step 8 .

STEP 8. Check oxygen sensor (front) itself.



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

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

STEP 9. Connector check: B-29 engine-ECU connector

Q. Is the check result normal?

    Check intermediate connector A-13, and repair if necessary. If intermediate connector is normal, check and repair harness between C-134 (terminal No. 3) oxygen sensor (front) connector and B-29 (terminal No. 38) engine-ECU connector.

    • Check output line for short circuit and damage.
Repair or replace the connector.

STEP 10. Connector check: B-29 engine-ECU connector

Q. Is the check result normal?

    Go to Step 6 .
Repair or replace the connector.