SPN 110 FMI 3 Detroit Diesel DD15 — Coolant Temp Faul (2026)
How We Tested and Verified
SPN 110 FMI 3 on Detroit Diesel 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 Detroit Diesel DD15 means engine coolant temperature - voltage above normal, circuit shorted high. 60–80% of field cases trace to open or failed coolant temperature sensor. Shop repair with parts runs $40 - $150; 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 Detroit Diesel DD15 tells the ECM that engine coolant temperature sensor circuit is outside calibrated limits. On J1939, FMI 3 specifies the failure mode: Engine Coolant Temperature - Voltage Above Normal, Circuit Shorted High. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. ProfessionalDieselRepair.com always confirms this is a wiring/sensor fault with a resistance check before letting a driver panic about overheating - the two conditions look identical on the dash but have very different fixes.
Common Symptoms
- Coolant temp gauge pegged at maximum or showing an implausible reading (100% of cases)
- Cooling fan runs constantly regardless of actual engine temperature (80% of cases)
- Amber check engine light (60% of cases)
- Possible slight fuel economy drop from conservative fueling maps (40% of cases)
Likely Causes (Ranked by Probability)
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Open or failed coolant temperature sensor (45% of cases) — Electrical fault - an internal open circuit in the thermistor produces a signal the ECM reads as impossibly high voltage.
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Corroded or disconnected sensor connector (25% of cases) — Electrical fault - a poor connection has the same effect as a fully open sensor circuit.
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Chafed harness wire shorted to a power source (15% of cases) — Electrical fault - a signal wire rubbing against a powered circuit near the valve cover can inject voltage directly onto the sensor line.
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Genuine coolant system issue causing extreme overheating (10% of cases) — A true mechanical failure - only relevant if actual coolant temperature is confirmed abnormally high by an independent thermometer, not just the dash gauge.
Step-by-Step Diagnostic Guide
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Connect Detroit Diesel Diagnostic Link (DDDL) and confirm the fault is active, then check the reported coolant temp value - a reading pegged at the sensor’s max scale confirms an open/shorted-high circuit rather than real overheating. — Tool: scan tool or multimeter. Document readings on the work order.
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Unplug the coolant temp sensor connector and measure resistance across the sensor pins - a typical thermistor should read roughly 2,000-3,000 ohms at 68°F, dropping as temperature rises. An open reading (infinite resistance) confirms a bad sensor or open circuit. — Tool: scan tool or multimeter. Document readings on the work order.
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With the sensor unplugged, check for battery voltage or a short to power on the signal wire back at the ECM connector. — Tool: scan tool or multimeter. Document readings on the work order.
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Inspect the harness routing near the thermostat housing and valve cover for heat damage or chafing against brackets. — Tool: scan tool or multimeter. Document readings on the work order.
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Clean and inspect the sensor connector pins for corrosion before reconnecting - a loose pin can present as an open circuit intermittently. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Engine Coolant Temperature Sensor Circuit wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Engine Coolant Temperature Sensor Circuit sensor/part | $40 - $150 | $$40 - $150 + labor | $300+ |
| Full module replacement | N/A | $600–$1,500 | $300+ |
Platform-Specific Notes
On Detroit Diesel 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 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 Detroit Diesel captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Engine Coolant Temperature Sensor 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
Assuming the engine is overheating and shutting it down on the highway shoulder unnecessarily — A pegged coolant gauge looks alarming and drivers react to the dash reading rather than the actual code.. Instead: This is a parts-cannon reaction in reverse - verify the actual coolant temperature with an infrared thermometer on the radiator before assuming a cooling system emergency.
Replacing the ECM for a coolant sensor circuit fault — When resistance checks seem inconclusive, some techs jump to the control module.. Instead: Recheck the sensor and full harness length with a proper resistance and voltage-drop test before ever considering the ECM as the cause.
Shop Notes from the Field
ProfessionalDieselRepair.com tip: this code almost always means an open sensor or connector, not an actual cooling system problem. Verify with a physical resistance check before you start chasing head gaskets or water pumps - the engine is very likely running at a normal temperature the whole time.
Safety Warning
- Let the engine cool before opening the cooling system to inspect anything physically.
- Verify actual coolant temperature independently before assuming the dash reading is accurate.
Prevention Tips
- Inspect coolant sensor connectors for corrosion during PM service.
- Route new harness repairs away from hot components like the valve cover and turbo.
- Address any coolant sensor code promptly rather than assuming it’s a false alarm every time.
Detroit Diesel Field Notes for SPN 110 FMI 3
- 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 circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Detroit Diesel.
- Capture a 20-minute data log of engine coolant temperature sensor 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 circuit mounting fasteners per Detroit Diesel 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 Detroit Diesel DD15 — cold-soak faults on engine coolant temperature sensor 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 circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Detroit Diesel.
- Capture a 20-minute data log of engine coolant temperature sensor circuit parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
Related Codes
- SPN 110 FMI 0 — often stacks with SPN 110 FMI 3
- SPN 111 FMI 1 — often stacks with SPN 110 FMI 3
FAQ
Can I keep driving with this code active?
Yes, in most cases - this is typically an electrical fault, not real overheating, but confirm actual coolant temperature to be safe before extended driving.
How do I know if the engine is really overheating?
Check the actual coolant temperature with an infrared thermometer at the radiator or thermostat housing - a normal reading confirms this is a sensor circuit fault.
Does this trigger a derate?
No, this specific fault doesn’t trigger an emissions derate, though the ECM may run a more conservative fan and fueling strategy as a precaution.
What does SPN 110 FMI 3 mean on Detroit Diesel DD15?
SPN 110 FMI 3 means engine coolant temperature - voltage above normal, circuit shorted high. The ECM detected engine coolant temperature sensor 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 $40 - $150. Dealer pricing runs $300+.
What is the most common cause of SPN 110 FMI 3?
Open or failed coolant temperature sensor — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 110 parameter definition
- Detroit Diesel OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys