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).
The engine-ECU applies an offset voltage of 0.5 V to the oxygen sensor (front) (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 (front) offset voltage is more than 0.4 V and is less than 0.6 V.
Battery positive voltage is more than 11 V and is less than 16.5 V.
Typically 60 seconds have passed since the engine starting sequence was completed.

Judgement Criterion

Oxygen sensor (front) output voltage is less 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 .

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-45 oxygen sensor (front) connector

Q. Is the check result normal?

	Go to Step 3 .

	Repair or replace the connector.

STEP 3. Perform voltage measurement at C-45 oxygen sensor (front) connector.

Disconnect connector, and measure at harness side.
Voltage between terminal No. 4 and earth.

OK: 0.4 - 0.6 V

Q. Is the check result normal?

	Go to Step 7 .

	Go to Step 4 .

STEP 4. Connector check: B-08 engine-ECU connector

Q. Is the check result normal?

	Go to Step 5 .

	Repair or replace the connector.

STEP 5. Check harness between C-45 (terminal No. 4) oxygen sensor (front) connector and B-08 (terminal No. 39) engine-ECU connector.

note

Before checking harness, check intermediate connector A-11, and repair if necessary.

Check earthing line for open circuit.

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 .

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. Connector check: B-08 engine-ECU connector

Q. Is the check result normal?

	Go to Step 8 .

	Repair or replace the connector.

STEP 8. Check harness between C-45 (terminal No. 4) oxygen sensor (front) connector and B-08 (terminal No. 39) engine-ECU connector.

note

Before checking harness, check intermediate connector A-11, and repair if necessary.

Check earthing line for damage.

Q. Is the check result normal?

	Go to Step 9 .

	Repair the damaged harness wire.

STEP 9. Perform voltage measurement at C-45 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 11 .

	Go to Step 10 .

STEP 10. Check oxygen sensor (front) itself.

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

Q. Is the check result normal?

	Check intermediate connector A-11, and repair if necessary. If intermediate connector is normal, check and repair harness between C-45 (terminal No. 3) oxygen sensor (front) connector and B-08 (terminal No. 38) engine-ECU connector. Check output line for short circuit.

	Replace oxygen sensor (front).

STEP 11. Check harness between C-45 (terminal No. 3) oxygen sensor (front) connector and B-08 (terminal No. 38) engine-ECU connector.

note

Before checking harness, check intermediate connector A-11, and repair if necessary.

Check output line for open circuit and damage.

Q. Is the check result normal?

	Go to Step 6 .

	Repair the damaged harness wire.