SPN 110 FMI 3 Freightliner DD15 — Coolant Temp Fault (2026)
How We Tested and Verified
SPN 110 FMI 3 on Freightliner DD15 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 110 FMI 3 on Freightliner DD15 means engine coolant temperature - voltage above normal, or shorted to high source. 60–80% of field cases trace to ect signal wire shorted to 5v or battery positive. Shop repair with parts runs $35 - $180; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.
What This Code Means
SPN 110 FMI 3 on Freightliner DD15 tells the ECM that engine coolant temperature sensor (ect) circuit is outside calibrated limits. On J1939, FMI 3 specifies the failure mode: Engine Coolant Temperature - Voltage Above Normal, or Shorted to High Source. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. A pegged-hot coolant gauge on a cold engine is a wiring problem until proven otherwise. ProfessionalDieselRepair.com walks Freightliner operators through pin-level checks on SPN 110 FMI 3 before they start throwing thermostats and water pumps at an electrical fault.
Common Symptoms
- Check engine lamp with SPN 110 FMI 3 active in JPRO or OEM software (100% of cases)
- Coolant temp gauge pegged hot or reading implausibly high at cold start (80% of cases)
- Radiator fan may run continuously or not at all depending on default strategy (60% of cases)
- DEF regen or DPF events may be inhibited with incorrect temp data (40% of cases)
- Fault may be present key-on with engine cold (25% of cases)
Likely Causes (Ranked by Probability)
-
ECT signal wire shorted to 5V or battery positive (45% of cases) — Rubbed harness near exhaust or battery cables pulls signal voltage high regardless of actual coolant temperature.
-
Failed coolant temperature sensor internal short (25% of cases) — Age and coolant contamination short the NTC element, sending a high voltage signal to the ECM.
-
Connector pin corrosion bridging supply and signal (15% of cases) — Moisture and glycol create conductive paths between pins at the sensor Deutsch connector.
Step-by-Step Diagnostic Guide
-
Connect Noregon JPRO and read ECT voltage on SPN 110; above 4.8V at cold soak confirms FMI 3. — Tool: scan tool or multimeter. Document readings on the work order.
-
Unplug the ECT sensor at the thermostat housing. Key on, measure signal pin voltage; above 0.5V unplugged indicates a harness short to power. — Tool: scan tool or multimeter. Document readings on the work order.
-
Verify 5.0V +/- 0.25V on the reference/supply pin with sensor disconnected. — Tool: scan tool or multimeter. Document readings on the work order.
-
Inspect harness along the firewall and near exhaust routing for melted insulation shorting signal to battery positive. — Tool: scan tool or multimeter. Document readings on the work order.
-
Replace sensor or repair harness; NTC sensors typically read 2-4k ohms at 77F pin-to-pin. Clear code and confirm temp tracks ambient after cold soak. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Engine Coolant Temperature Sensor (ECT) Circuit wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Engine Coolant Temperature Sensor (ECT) Circuit sensor/part | $35 - $180 | $$35 - $180 + labor | $300+ |
| Full module replacement | N/A | $600–$1,500 | $300+ |
Platform-Specific Notes
On Freightliner DD15, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.
When This Code Sets
SPN 110 FMI 3 sets when engine coolant temperature sensor (ect) circuit fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Freightliner captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Engine Coolant Temperature Sensor (ECT) Circuit live data PID at idle, cruise, and full load
- Battery voltage and ECM supply voltage
- Related aftertreatment and protection PIDs (SPN 1569 if stacked)
Common Misdiagnoses
Replacing the water pump or thermostat for a voltage FMI 3 — The dash shows overheating.. Instead: Parts-cannon cooling repairs waste money. If the engine is cold to the touch and voltage is high, the circuit is lying.
Ignoring the code because the truck is not overheating — No steam or boil-over visible.. Instead: Default temp values can block regens and affect fan operation. Repair the circuit promptly.
Shop Notes from the Field
ProfessionalDieselRepair.com gets Freightliner SPN 110 FMI 3 tickets after PMs where the ECT connector was left loose. A shorted signal wire makes the dash read overheating when the block is cold; do not open the radiator based on gauge alone.
Safety Warning
- Never remove a hot radiator cap based on gauge reading alone.
- Disconnect battery before repairing harness shorts near positive cables.
- Allow engine to cool before working at the thermostat housing.
Prevention Tips
- Route harness clips away from exhaust manifolds during any radiator or turbo work.
- Replace ECT sensors when coolant is contaminated from oil or fuel.
- Inspect connector seals at every cooling system service.
Freightliner Field Notes for SPN 110 FMI 3
- Torque engine coolant temperature sensor (ect) circuit mounting fasteners per Freightliner OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on Freightliner DD15 — cold-soak faults on engine coolant temperature sensor (ect) circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 110 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If engine coolant temperature sensor (ect) circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Freightliner.
- Capture a 20-minute data log of engine coolant temperature sensor (ect) circuit parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 110 FMI 3 on DD15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque engine coolant temperature sensor (ect) circuit mounting fasteners per Freightliner OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on Freightliner DD15 — cold-soak faults on engine coolant temperature sensor (ect) circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 110 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If engine coolant temperature sensor (ect) circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Freightliner.
- Capture a 20-minute data log of engine coolant temperature sensor (ect) circuit parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 110 FMI 3 on DD15 — many bulletin fixes address harness routing rather than sensor replacement.
Related Codes
- SPN 110 FMI 4 — often stacks with SPN 110 FMI 3
- SPN 110 FMI 0 — often stacks with SPN 110 FMI 3
- SPN 111 FMI 3 — often stacks with SPN 110 FMI 3
FAQ
What does Freightliner SPN 110 FMI 3 mean?
Coolant temperature sensor voltage is above the ECM expected range, usually a short to a high voltage source.
Can this code cause overheating?
The code itself is electrical, but incorrect temp data can prevent the fan from engaging when truly needed.
How do I test with the sensor unplugged?
Key on, signal pin should read near 0V. Above 0.5V means a harness short to power.
What does SPN 110 FMI 3 mean on Freightliner DD15?
SPN 110 FMI 3 means engine coolant temperature - voltage above normal, or shorted to high source. The ECM detected engine coolant temperature sensor (ect) circuit outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 110 FMI 3 active?
Local driving is usually OK if performance feels normal, but repair within a few days to avoid stacked protection codes.
How much does it cost to fix SPN 110 FMI 3?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $35 - $180. Dealer pricing runs $300+.
What is the most common cause of SPN 110 FMI 3?
ECT signal wire shorted to 5V or battery positive — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 110 parameter definition
- Freightliner OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys