Coolant Temperature Sensor (ECT)
Location, connector pinout, test procedure, and related fault codes for the engine coolant temperature sensor on Class 8 diesel trucks.
Last updated: 2026-09-20
Overview
The engine coolant temperature (ECT) sensor monitors coolant temperature for cold-start fueling, fan control, EGR strategy, and overtemperature protection. A failed ECT sensor can cause hard cold starts, excessive idle time, or missed overtemperature shutdowns.
Sensor Locations by Platform
Cummins X15
Threaded into coolant passage on driver side of engine block or cylinder head, near thermostat housing.
X15 may have two coolant temp sensors — one for ECM and one for gauge. Verify which is faulting.
PACCAR MX-13
Driver side of engine block, near thermostat housing. Threaded into coolant passage.
MX-13 ECT sensor is accessible from the driver side.
Detroit DD15
Driver side, threaded into coolant passage near front of engine. Near thermostat housing.
DD15 coolant temp sensor is a common fault — inspect connector for corrosion.
Volvo/Mack MP8
Driver side of engine block, near thermostat housing.
Verify coolant level before condemning sensor — air pockets cause false low readings.
DieselVision™ identifies sensor labels and connector pin numbers — upload a close-up photo and the system maps pins automatically.
Connector Pinout
| Pin | Function | Specification |
|---|---|---|
| A | Signal (Temperature) | High resistance (cold) to low resistance (hot). ~2.5V at 25°C, drops as temp rises |
| B | Ground (Signal Return) | <0.1V to chassis ground |
Test Procedure
Read coolant temperature live data before engine start
Expected: Should match ambient temperature when cold. If reading -40°C = open circuit. If reading 130°C = short to ground
Tool: OEM diagnostic software
Measure sensor resistance across pins A and B (sensor disconnected, key off)
Expected: At 25°C: ~2,000–3,000 ohms. At operating temp (90°C): ~200–300 ohms. Infinite = open circuit
Tool: Multimeter (ohms)
Monitor coolant temperature during warm-up
Expected: Temperature should rise steadily from ambient to ~85–95°C operating temperature. Stuck reading = failed sensor
Tool: OEM diagnostic software
Check ground circuit resistance from pin B to chassis ground
Expected: <0.5 ohms
Tool: Multimeter
Verify actual coolant temperature with infrared thermometer on coolant hose
Expected: Infrared reading should be within 10°C of sensor reading. Large discrepancy = sensor fault
Tool: Infrared thermometer
Common Failure Modes
- •Open circuit — sensor reads -40°C (minimum value)
- •Short to ground — sensor reads maximum temperature
- •Internal thermistor failure — stuck reading
- •Connector corrosion causing high resistance
- •Sensor installed in air pocket — reads lower than actual coolant temp
Frequently Asked Questions
Where is the coolant temperature sensor located on a diesel engine?
The coolant temperature sensor is threaded into a coolant passage on the driver side of the engine block or cylinder head, near the thermostat housing.
What does a coolant temp sensor reading of -40°C mean?
A reading of -40°C (minimum value) indicates an open circuit in the sensor or signal wire. Check sensor resistance and wiring continuity.
How do I test a coolant temperature sensor?
Disconnect the sensor and measure resistance across the two pins. At room temperature (25°C) you should see approximately 2,000–3,000 ohms. Resistance should decrease as temperature increases.
What fault codes are caused by a failed coolant temp sensor?
Common fault codes include SPN 110 FMI 0 (coolant temp above normal — critical), SPN 110 FMI 3 (voltage above normal), and SPN 110 FMI 4 (voltage below normal / open circuit).
Snap a photo of your scan tool screen and let Diesel Vision interpret the fault instantly — no manual code lookup required.
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