Code No. P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

OPERATION

A power voltage of 5 V is applied to the engine coolant temperature sensor power terminal from the engine-ECU connector terminal WTS.
The power voltage is earthed to the engine-ECU connector terminal WTSE from the engine coolant temperature sensor connector terminal.

FUNCTION

The engine coolant temperature sensor converts the engine coolant temperature into a voltage signal, and inputs the voltage to the engine-ECU.
In response to the signal, the engine-ECU controls the fuel injection amount and the fast idle speed when the engine is cold state.
The engine coolant temperature sensor is a kind of resistor, which has characteristics to reduce its resistance as the engine coolant temperature rises. Therefore, the sensor output voltage varies with the engine coolant temperature, and becomes lower as the engine coolant temperature rises.

TROUBLE JUDGMENT

Check Conditions, Judgement Criteria

While engine is running.
Engine coolant temperature decreases from more than 40°C to less than 40°C.
Then the engine coolant temperature is less than 40°C for 5 minutes.

FAIL-SAFE AND BACKUP FUNCTION

Control as if the engine coolant temperature is 80°C.

PROBABLE CAUSES

Failed engine coolant temperature sensor
Harness damage in engine coolant temperature sensor circuit or loose connector contact
Failed engine-ECU

DIAGNOSIS PROCEDURE

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

Item 6: Engine coolant temperature sensor

OK:
Engine cold state: At ambient temperature (atmospheric temperature) or equivalent.
Engine hot state: At 80 - 120°C

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 resistance measurement at engine coolant temperature sensor connector.

Disconnect connector, and measure at sensor side.
Resistance between terminals.

OK:
Engine coolant temperature at -20°C: 14 - 17 kΩ
Engine coolant temperature at 0°C: 5.1 - 6.5 kΩ
Engine coolant temperature at 20°C: 2.1 - 2.7 kΩ
Engine coolant temperature at 40°C: 0.9 - 1.3 kΩ
Engine coolant temperature at 60°C: 0.48 - 0.68 kΩ
Engine coolant temperature at 80°C: 0.26 - 0.36 kΩ

Q. Is the check result normal?

	Go to Step 3.

	Repair or replace the connector, or replace the engine coolant temperature sensor.

STEP 3. Perform voltage measurement at engine coolant temperature sensor connector.

Use special tool test harness (MB991709) to connect connector, and measure at pick-up harness.
Ignition switch: "ON"
Voltage between terminal engine-ECU WTS line and body earth.

OK:
Engine coolant temperature at -20°C: 3.9 - 4.5 V
Engine coolant temperature at 0°C: 3.2 - 3.8 V
Engine coolant temperature at 20°C: 2.3 - 2.9 V
Engine coolant temperature at 40°C: 1.3 - 1.9 V
Engine coolant temperature at 60°C: 0.7 - 1.3 V
Engine coolant temperature at 80°C: 0.3 - 0.9 V

Q. Is the check result normal?

	Go to Step 5.

	Go to Step 4.

STEP 4. Check harness between engine coolant temperature sensor connector terminal and engine-ECU connector terminal WTS.

Check power line for damage.

Q. Is the check result normal?

	Go to Step 5.

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

STEP 5. Check harness between engine coolant temperature sensor connector terminal and engine-ECU connector terminal WTSE.

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. M.U.T.-III data list

Item 6: Engine coolant temperature sensor

OK:
Engine cold state: At ambient temperature (atmospheric temperature) or equivalent.
Engine hot state: At 80 - 120 °C

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.