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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 from the ECM terminal WTS via the resistor in the ECM. The ground terminal is grounded with ECM terminal SENG.
  • 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 0°C (32°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)
  • Mass airflow sensor
  • Intake air temperature sensor
 
 
DTC SET CONDITIONS <Range/Performance problem - drift>
Logic Flow Chart
 
 
Check Conditions, Judgment Criteria
  • Engine coolant temperature decreases from more than 40°C (104°F) to less than 40°C (104°F).
  • Then the engine coolant temperature is less than 40°C (104°F) for 5 minutes.
  • However, time is not counted when fuel is shut off.
 
 
DTC SET CONDITIONS <Range/Performance problem - low input (Time to reach closed loop temperature)>
Logic Flow Chart
Check Conditions, Judgment Criteria
  • About 90 - 300 seconds have passed for the engine coolant temperature to rise to about 0°C (32°F) after starting sequence was completed.
  • However, time is not counted when fuel is shut off.
 
 
FAIL-SAFE AND BACKUP FUNCTION
  • Control as if the engine coolant temperature is 80°C (176°F).
 
 
OBD-II DRIVE CYCLE PATTERN
  • Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 8 .
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:
  • MB992744: Vehicle communication interface-Lite (V.C.I.-Lite)
  • MB992745: V.C.I.-Lite main harness A
  • MB992747: V.C.I.-Lite USB cable short
  • MB992748: V.C.I.-Lite USB cable long
  • MB991958: Scan Tool (M.U.T.-III Sub Assembly)
    • MB991824: Vehicles Communication Interface (V.C.I.)
    • MB991827: M.U.T.-III USB Cable
    • MB991910: M.U.T.-III Main Harness A (Vehicles with CAN communication system)
STEP 1. Using scan tool (M.U.T.-III), check data list item 6: Engine Coolant Temperature Sensor.
cautionTo 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 (M.U.T.-III). Refer to HOW TO CONNECT THE SCAN TOOL (M.U.T.-III) .
(2) Turn the ignition switch to the "ON" position.
(3) Set scan tool (M.U.T.-III) 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.
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 by backprobing.
(1) Do not disconnect the connector.
(2) Turn the ignition switch to the "ON" position.
(3) Measure the voltage between terminal WTS line 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.
Is the measured voltage within the specified range?
Go to Step 3.
Go to Step 4.
 
STEP 3. Using scan tool (M.U.T.-III), check data list item 6: Engine Coolant Temperature Sensor.
(1) Turn the ignition switch to the "ON" position.
(2) Set scan tool (M.U.T.-III) 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.
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 (Refer to, Removal and Installation ). Then go to Step 9.
 
STEP 4. Measure the sensor supply voltage at engine coolant temperature sensor harness side connector.
(1) Disconnect the connector and measure at the harness side.
(2) Turn the ignition switch to the "ON" position.
(3) Measure the voltage between terminal WTS line and ground.
  • Voltage should be between 4.5 and 4.9 volts.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the measured voltage between 4.5 and 4.9 volts?
Go to Step 5.
Replace the ECM (Refer to, Removal and Installation ). Then go to Step 9.
 
STEP 5. Check the continuity at engine coolant temperature sensor harness side connector.
(1) Disconnect the connector and measure at the harness side.
(2) Check for the continuity between terminal SENG and ground.
  • Continuity (2 ohms or less)
Does continuity exist?
Go to Step 7.
Go to Step 6.
 
STEP 6. Check of damage in SENG line between engine coolant temperature sensor connector and ECM connector.
Is the harness wire in good condition?
Replace the ECM (Refer to, Removal and Installation ). Then go to Step 9.
Repair it. Then go to Step 9.
 
STEP 7. Check the engine coolant temperature sensor.
Refer to Engine Coolant Temperature Sensor Check .
Is the engine coolant temperature sensor normal?
Go to Step 8.
Replace the engine coolant temperature sensor. Then go to Step 9.
 
STEP 8. Check of damage in WTS line between engine coolant temperature sensor connector and ECM connector.
Is the harness wire in good condition?
Replace the ECM (Refer to, Removal and Installation ). Then go to Step 9.
Repair it. Then go to Step 9.
 
STEP 9. 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 8 .
(2) Check the diagnostic trouble code (DTC).
Is DTC P0125 set?
Retry the troubleshooting.
The inspection is complete.