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 output terminal (terminal No. 1) from the engine-A/T-ECU (terminal No. 98).
The power voltage is earthed to the engine-A/T-ECU (terminal No. 96) from the engine coolant temperature sensor (terminal No. 2).

FUNCTION

The engine coolant temperature sensor converts the engine coolant temperature into a voltage signal, and inputs the voltage to the engine-A/T-ECU.
In response to the signal, the engine-A/T-ECU controls the fuel infection 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 Condition

While engine is running.

Judgement Criterion

Engine coolant temperature decreases from higher than 40°C to lower than 40°C.

PROBABLE CAUSES

Failed engine coolant temperature sensor
Open/short circuit in engine coolant temperature sensor circuit or loose connector contact
Failed engine-A/T-ECU

DIAGNOSIS PROCEDURE

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

Item 06: 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. Connector check: B-26 engine coolant temperature sensor connector

Q. Is the check result normal?

	Go to Step 3 .

	Repair or replace the connector.

STEP 3. Perform resistance measurement at B-26 engine coolant temperature sensor connector.

Disconnect connector, and measure at sensor side.
Resistance between terminal No. 1 and No. 2.

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 4 .

	Replace the engine coolant temperature sensor.

STEP 4. Perform voltage measurement at B-26 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 No. 1 and 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 7 .

	Go to Step 5 .

STEP 5. Connector check: D-217 engine-A/T-ECU connector

Q. Is the check result normal?

	Go to Step 6 .

	Repair or replace the connector.

STEP 6. Check harness between B-26 (terminal No. 1) engine coolant temperature sensor connector and D-217 (terminal No. 98) engine-A/T-ECU connector.

Check output line for damage.

Q. Is the check result normal?

	Go to Step 7 .

	Repair the damaged harness wire.

STEP 7. M.U.T.-III data list

Item 06: 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-A/T-ECU.