Code No. P0131: Oxygen Sensor (front) Circuit Low Voltage <EURO2>

OPERATION

The sensor signal is inputted to the engine-ECU connector terminal O2FL from the oxygen sensor (front) output terminal.
The oxygen sensor (front) is earthed with engine-ECU connector terminal O2VE.
The engine-ECU connector terminal O2VE applies an offset voltage of 0.5 V to the oxygen sensor (front).

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 JUDGEMENT

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.

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

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. Voltage measurement at oxygen sensor (front) connector (O2VE line).

(1)Disconnect the connector, and measure at the wiring harness side.

(2)Ignition switch: "ON"

(3)Check the voltage between the oxygen sensor (front) connector (O2VE 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 of short to earth and open circuit in O2VE line between oxygen sensor (front) connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 4.

	Repair the connector(s) or wiring harness.

STEP 4. 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 5. Check of harness damage in O2VE line between oxygen sensor (front) connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 6.

	Repair the connector(s) or wiring harness.

STEP 6. Voltage measurement at oxygen sensor (front) connector (O2FL line).

(1)Use special tool test harness (MB991658) to connect connector, and measure at pick-up harness.

(2)Engine: After warm-up

(3)Transmission: P range

(4)Check the voltage between the oxygen sensor (front) connector (O2FL line) and body 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) check (Refer to ).

Q. Is the check result normal?

	Repair or replace the connector(s), or check and repair the harness of short circuit to earth in O2FL line between oxygen sensor (front) connector and engine-ECU connector.

	Replace the oxygen sensor (front).

STEP 8. Check of open circuit and damage in O2FL line between oxygen sensor (front) connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 4.

	Repair the connector(s) or wiring harness.