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

OPERATION

The sensor signal is inputted to the engine-ECU connector terminal O2FL from the oxygen sensor (front) output terminal.
The oxygen sensor (front) connector terminal is earthed with engine-ECU connector terminal OFLE.
The engine-ECU connector terminal OFLE applies an offset voltage of 0.5 V to the oxygen sensor (front) connector terminal O2FL.

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

More than 1 minute have passed since the engine starting sequence was completed. (when engine coolant temperature is 20°C or more during engine start)
Battery positive voltage is more than 11 V and is less than 16.5 V.
Oxygen sensor (front) offset voltage is more than 0.4 V and is less than 0.6 V.

Judgement Criterion

Oxygen sensor (front) output voltage is less than 0.2 V for 2 seconds.

FAIL-SAFE AND BACKUP FUNCTION

None

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

Data list reference table (Refer to ).

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. Perform voltage measurement at oxygen sensor (front) connector.

Disconnect connector, and measure at harness side.
Ignition switch: "ON"
Voltage between terminal engine-ECU OFLE line and body earth.

OK: 0.4 - 0.6 V

Q. Is the check result normal?

	Go to Step 5.

	Go to Step 3.

STEP 3. Check harness between oxygen sensor (front) connector terminal and engine-ECU connector terminal OFLE.

Check earthing line for open/short circuit.

Q. Is the check result normal?

	Go to Step 4.

	Repair or replace the connector, or repair the damaged harness wire.

STEP 4. M.U.T.-III data list

Data list reference table (Refer to ).

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 5. Check harness between oxygen sensor (front) connector terminal and engine-ECU connector terminal OFLE.

Check earthing line for damage.

Q. Is the check result normal?

	Go to Step 6.

	Repair or replace the connector, or repair the damaged harness wire.

STEP 6. Perform voltage measurement at 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 engine-ECU O2FL line and earth.

OK:
When the engine is 2,500 r/min, the output voltage should repeat 0.9 V or less <=> 1.1 to 1.5 V alternately.

Q. Is the check result normal?

	Go to Step 8.

	Go to Step 7.

STEP 7. Check oxygen sensor (front) itself.

Oxygen sensor (front) connector check.
Oxygen sensor check (Refer to ).

Q. Is the check result normal?

	Repair or replace the connector, or check and repair harness between oxygen sensor (front) connector terminal and engine-ECU connector terminal O2FL. Check output line for short circuit.

	Repair or replace the connector, or replace oxygen sensor (front).

STEP 8. Check harness between oxygen sensor (front) connector terminal and engine-ECU connector terminal O2FL.

Check output line for open circuit and damage.

Q. Is the check result normal?

	Go to Step 4.

	Repair or replace the connector, or repair the damaged harness wire.