DTC P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

CIRCUIT OPERATION

5-volt voltage is applied to the engine coolant temperature sensor output terminal (terminal No. 1) from the ECM (terminal No. 26) via the resistor in the ECM. The ground terminal (terminal No. 2) is grounded with ECM (terminal No. 27).
The engine coolant temperature sensor is a negative temperature coefficient type of resistor. It has the characteristic that when the engine coolant temperature rises the resistance decreases.
The engine coolant temperature sensor output voltage increases when the resistance increases and decreases when the resistance decreases.

TECHNICAL DESCRIPTION

The engine coolant temperature sensor converts the engine coolant temperature to a voltage and outputs it.
The ECM checks whether this voltage is within a specified range.

DESCRIPTIONS OF MONITOR METHODS

Engine coolant temperature sensor output voltage drops from over 40°C (104°F) to under 40°C (104°F) and keeps under 40°C (104°F) for 5 minutes.
Engine coolant temperature sensor output voltage does not reach closed loop enable temperature within specified period when engine coolant temperature sensor output voltage at engine start is under 7°C (45°F).

MONITOR EXECUTION

MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)

Other Monitor (There is no temporary DTC stored in memory for the item monitored below)

Not applicable

Sensor (The sensor below is determined to be normal)

Mass airflow sensor
Intake air temperature sensor

DTC SET CONDITIONS <Range/Performance problem - drift>

Logic Flow Chart

Check Conditions, Judgement Criteria

Engine coolant temperature decreases from higher than 40°C (104°F) to lower than 40°C (104°F).
Then the engine coolant temperature has continued to be 40°C (104°F) or lower for 5 minutes.

DTC SET CONDITIONS <Range/Performance problem - low input (Time to reach closed loop temperature)>

Logic Flow Chart

Check Conditions, Judgement Criteria

About 90 - 300 seconds have passed for the engine coolant temperature to rise to about 7°C (45°F) after starting sequence was completed.
However, time is not counted when fuel is shut off.

OBD-II DRIVE CYCLE PATTERN

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)

Engine coolant temperature sensor failed.
Harness damage in engine coolant temperature sensor circuit or connector damage.
ECM failed.

DIAGNOSIS

Required Special Tools:

MB991958: Scan Tool (M.U.T.-III Sub Assembly)

MB991824: V.C.I.
MB991827: USB Cable
MB991910: Main Harness A

STEP 1. Using scan tool MB991958, check data list item 6: Engine Coolant Temperature Sensor.

caution

To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

(1)Connect scan tool MB991958 to the data link connector.
(2)Turn the ignition switch to the "ON" position.
(3)Set scan tool MB991958 to the data reading mode for item 6, Engine Coolant Temperature Sensor.

The engine coolant temperature and temperature shown with the scan tool should approximately match.

(4)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the sensor operating properly?

It can be assumed that this malfunction is intermittent. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions

Go to Step 2.

STEP 2. Measure the sensor output voltage at engine coolant temperature sensor connector B-06 by backprobing.

(1)Do not disconnect the connector B-06.

(2)Turn the ignition switch to the "ON" position.

(3)Measure the voltage between terminal No. 1 and ground by backprobing.

When engine coolant temperature is -20°C (-4°F), voltage should be between 3.9 and 4.5 volts.
When engine coolant temperature is 0°C (32°F), voltage should be between 3.2 and 3.8 volts.
When engine coolant temperature is 20°C (68°F), voltage should be between 2.3 and 2.9 volts.
When engine coolant temperature is 40°C (104°F), voltage should be between 1.3 and 1.9 volts.
When engine coolant temperature is 60°C (140°F), voltage should be between 0.7 and 1.3 volts.
When engine coolant temperature is 80°C (176°F), voltage should be between 0.3 and 0.9 volt.

(4)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the measured voltage within the specified range?

Go to Step 3.

Go to Step 5.

STEP 3. Check harness connector B-06 at the engine coolant temperature sensor for damage.

Q. Is the harness connector in good condition?

	Go to Step 4.

	Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then go to Step 14.

STEP 4. Using scan tool MB991958, check data list item 6: Engine Coolant Temperature Sensor.

(1)Turn the ignition switch to the "ON" position.

(2)Set scan tool MB991958 to the data reading mode for item 6, Engine Coolant Temperature Sensor.

The engine coolant temperature and temperature shown with the scan tool should approximately match.

(3)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the sensor operating properly?

	It can be assumed that this malfunction is intermittent. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions .

	Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP 42B, Diagnosis - ID Code Registration Judgment Table <Vehicles with KOS> or GROUP 42C, Diagnosis - ID Codes Registration Judgment Table <Vehicles with WCM> . Then go to Step 14.

STEP 5. Check harness connector B-06 at engine coolant temperature sensor for damage.

Q. Is the harness connector in good condition?

	Go to Step 6.

	Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then go to Step 14.

STEP 6. Measure the sensor supply voltage at engine coolant temperature sensor harness side connector B-06.

(1)Disconnect the connector B-06 and measure at the harness side.

(2)Turn the ignition switch to the "ON" position.

(3)Measure the voltage between terminal No. 1 and ground.

Voltage should be between 4.5 and 4.9 volts.

(4)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the measured voltage between 4.5 and 4.9 volts?

Go to Step 8.

Go to Step 7.

STEP 7. Check harness connector B-108 at ECM for damage.

Q. Is the harness connector in good condition?

	Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP 42B, Diagnosis - ID Code Registration Judgment Table <Vehicles with KOS> or GROUP 42C, Diagnosis - ID Codes Registration Judgment Table <Vehicles with WCM> . Then go to Step 14.

	Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then go to Step 14.

STEP 8. Check the continuity at engine coolant temperature sensor harness side connector B-06.

(1)Disconnect the connector B-06 and measure at the harness side.

(2)Check for the continuity between terminal No. 2 and ground.

Continuity (2 Ω or less)

Q. Does continuity exist?

Go to Step 11.

Go to Step 9.

STEP 9. Check harness connector B-108 at ECM for damage.

Q. Is the harness connector in good condition?

	Go to Step 10.

	Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then go to Step 14.

STEP 10. Check for harness damage between engine coolant temperature sensor connector B-06 (terminal No. 2) and ECM connector B-108 (terminal No. 27).

Q. Is the harness wire in good condition?

	Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP 42B, Diagnosis - ID Code Registration Judgment Table <Vehicles with KOS> or GROUP 42C, Diagnosis - ID Codes Registration Judgment Table <Vehicles with WCM> . Then go to Step 14.

	Repair it. Then go to Step 14.

STEP 11. Check the engine coolant temperature sensor.

Refer to Engine Coolant Temperature Sensor

Q. Is the engine coolant temperature sensor normal?

	Go to Step 12.

	Replace the engine coolant temperature sensor. Then go to Step 14.

STEP 12. Check harness connector B-108 at ECM for damage.

Q. Is the harness connector in good condition?

	Go to Step 13.

	Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then go to Step 14.

STEP 13. Check for harness damage between engine coolant temperature sensor connector B-06 (terminal No. 1) and ECM connector B-108 (terminal No. 26).

Q. Is the harness wire in good condition?

	Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP 42B, Diagnosis - ID Code Registration Judgment Table <Vehicles with KOS> or GROUP 42C, Diagnosis - ID Codes Registration Judgment Table <Vehicles with WCM> . Then go to Step 14.

	Repair it. Then go to Step 14.

STEP 14. Test the OBD-II drive cycle.

(1)Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 9

(2)Check the diagnostic trouble code (DTC).

Q. Is DTC P0125 set?

	Retry the troubleshooting.

	The inspection is complete.