P0125

DTC P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

Enging Coolant Temperature Sensor Circuit:

CIRCUIT OPERATION
- 5-volt voltage is applied to the engine coolant temperature sensor output terminal (terminal No.1) from the ECM (terminal No.44) via the resistor in the ECM. The ground terminal (terminal No.2) is grounded with ECM (terminal No.49).
- 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 degrees C (104 degrees F) to under 40 degrees C (104 degrees F) and keeps under 40 degrees C (104 degrees F) for 5 minutes.
- Engine coolant temperature sensor output voltage does not reach close loop enable temperature within specified period when engine coolant temperature sensor output voltage at engine start is under 7 degrees C (45 degrees F).

MONITOR EXECUTION
- Continuous

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)
- Volume airflow sensor
- Engine coolant temperature sensor
- Intake air temperature sensor

DTC SET CONDITIONS

Logic Flow Chart:

Logic Flow Chart

Check Conditions, Judgment Criteria
- About 60 - 300 seconds have passed for the engine coolant temperature to rise to about 7 degrees C (45 degrees F) after starting sequence was completed.
- However, time is not counted when fuel is shut off.

DTC SET CONDITIONS

Logic Flow Chart:

Logic Flow Chart

Check Conditions, Judgment Criteria
- Engine coolant temperature decreases from higher than 40 degrees C (104 degrees F) to lower than 40 degrees C (104 degrees F).
- Then the engine coolant temperature has continued to be 40 degrees C (104 degrees F) or lower for 5 minutes.

OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 9. OBD-II Drive Cycle Pattern Lists

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
- MB991911: Main Harness B

STEP 1. Using scan tool MB991958, check data list item 21: 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 21, 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?

YES: It can be assumed that this malfunction is intermittent.

NO: Go to Step 2.

STEP 2. Measure the sensor output voltage at engine coolant temperature sensor connector B-107 by backprobing.

1. Do not disconnect connector B-107.
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 degree C (-4 degree F), voltage should be between 3.9 and 4.5 volts.
- When engine coolant temperature is 0 degree C (32 degrees F), voltage should be between 3.2 and 3.8 volts.
- When engine coolant temperature is 20 degrees C (68 degrees F), voltage should be between 2.3 and 2.9 volts.
- When engine coolant temperature is 40 degrees C (104 degrees F), voltage should be between 1.3 and 1.9 volts.
- When engine coolant temperature is 60 degrees C (140 degrees F), voltage should be between 0.7 and 1.3 volts.
- When engine coolant temperature is 80 degrees C (176 degrees 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?

YES: Go to Step 3.

NO: Go to Step 5.

STEP 3. Check harness connector B-107 at the engine coolant temperature sensor for damage.

Q: Is the harness connector in good condition?

YES: Go to Step 4.

NO: Repair or replace it. Then go to Step 14.

STEP 4. Using scan tool MB991958, check data list item 21: 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.

STEP 5. Check harness connector B-107 at engine coolant temperature sensor for damage.

Q: Is the harness connector in good condition?

YES: Go to Step 6.

NO: Repair or replace it. Then go to Step 14.

STEP 6. Measure the sensor supply voltage at engine coolant temperature sensor harness side connector B-107.

1. Disconnect connector B-107 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?

YES: Go to Step 8.

NO: Go to Step 7.

STEP 7. Check harness connector C-119 at ECM for damage.

Q: Is the harness connector in good condition?

YES: Replace the ECM. Then go to Step 14.

NO: Repair or replace it. Then go to Step 14.

STEP 8. Check for continuity at engine coolant temperature sensor harness side connector B-107.

1. Disconnect connector B-107 and measure at the harness side.

2. Check for the continuity between terminal No.2 and ground.
- Should be less than 2 Ohms.

Q: Does continuity exist?

YES: Go to Step 11.

NO: Go to Step 9.

STEP 9. Check harness connector C-119 at ECM for damage.

Q: Is the harness connector in good condition?

YES: Go to Step 10.

NO: Repair or replace it. Then go to Step 14.

STEP 10. Check for harness damage between engine coolant temperature sensor connector B-107 (terminal No.2) and ECM connector C-119 (terminal No.49).

Q: Is the harness wire in good condition?

YES: Replace the ECM. Then go to Step 14.

NO: Repair it. Then go to Step 14.

STEP 11. Check the engine coolant temperature sensor.

1. Disconnect the engine coolant temperature sensor connector B-107.
2. Remove the engine coolant temperature sensor.

3. With the temperature sensing portion of engine coolant temperature sensor immersed in hot water, check resistance.

Standard value:
14 - 17 kOhms [at -20 degree C (-4 degree F)]
5.1 - 6.5 kOhms [at 0 degree C (32 degrees F)]
2.1 - 2.7 kOhms [at 20 degrees C (68 degrees F)]
0.9 - 1.3 kOhms [at 40 degrees C (104 degrees F)]
0.48 - 0.68 kOhm [at 60 degrees C (140 degrees F)]
0.26 - 0.36 kOhm [at 80 degrees C (176 degrees F)]

4. Apply 3M(TM) AAD part number 8731 or equivalent on the screw section of the engine coolant temperature sensor.
5. Install the engine coolant temperature sensor, and tighten to the specified torque.

Tightening torque: 29 ± 10 N.m (22 ± 6 ft-lb)

Q: Is the measured resistance at the standard value?

YES: Go to Step 12.

NO: Replace the engine coolant temperature sensor. Then go to Step 14.

STEP 12. Check harness connector C-119 at ECM for damage.

Q: Is the harness connector in good condition?

YES: Go to Step 13.

NO: Repair or replace it. Then go to Step 14.

STEP 13. Check for harness damage between engine coolant temperature sensor connector B-107 (terminal No.1) and ECM connector C-119 (terminal No.44).

Q: Is the harness wire in good condition?

YES: Replace the ECM. Then go to Step 14.

NO: 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. OBD-II Drive Cycle Pattern Lists
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0125 set?

YES: Retry the troubleshooting.

NO: The inspection is complete.