Detroit Diesel SPN 110 FMI 3: Coolant Temp Voltage Above Normal
SPN 110 FMI 3 (FMI 3) — Engine Coolant Temperature - Voltage Above Normal, Circuit Shorted High
Quick Answer
SPN 110 FMI 3 means the coolant temperature sensor circuit is shorted high or open, not that the engine is actually overheating. It's Medium severity - an electrical fault that pushes the ECM into a conservative fan and fueling strategy until repaired.
Overview: Detroit Diesel SPN 110 FMI 3
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.
Repair Snapshot
Time
45 Minutes - 1.5 Hours
Difficulty
Beginner
Parts Cost
$40 - $150
Dealer Labor
$300+
Tools Required
- Detroit Diesel Diagnostic Link (DDDL)
- Multimeter
- Jaltest
What SPN 110 FMI 3 Means
SPN 110 FMI 3 means the coolant temperature circuit is reading a voltage higher than physically possible, which the ECM interprets as a short to a high-voltage source or an open sensor ground. This is Medium severity - it's an electrical signal fault, not an actual overheating condition, but the ECM will default to a conservative fueling/fan strategy until it's fixed.
Symptoms Drivers Notice
- Coolant temp gauge pegged at maximum or showing an implausible reading
- Cooling fan runs constantly regardless of actual engine temperature
- Amber check engine light
- Possible slight fuel economy drop from conservative fueling maps
Most Likely Root Causes
Open or failed coolant temperature sensor
CommonElectrical fault - an internal open circuit in the thermistor produces a signal the ECM reads as impossibly high voltage.
Corroded or disconnected sensor connector
CommonElectrical fault - a poor connection has the same effect as a fully open sensor circuit.
Chafed harness wire shorted to a power source
OccasionalElectrical fault - a signal wire rubbing against a powered circuit near the valve cover can inject voltage directly onto the sensor line.
Genuine coolant system issue causing extreme overheating
RareA true mechanical failure - only relevant if actual coolant temperature is confirmed abnormally high by an independent thermometer, not just the dash gauge.
Common Misdiagnoses (Don't Waste Parts)
Assuming the engine is overheating and shutting it down on the highway shoulder unnecessarily
Why it happens: A pegged coolant gauge looks alarming and drivers react to the dash reading rather than the actual code.
Do this 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
Why it happens: When resistance checks seem inconclusive, some techs jump to the control module.
Do this instead: Recheck the sensor and full harness length with a proper resistance and voltage-drop test before ever considering the ECM as the cause.
Master Tech Commentary
Mechanic's Notes
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.
Step-by-Step Diagnostic Procedure
- Step 1: 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.
- Step 2: 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.
- Step 3: With the sensor unplugged, check for battery voltage or a short to power on the signal wire back at the ECM connector.
- Step 4: Inspect the harness routing near the thermostat housing and valve cover for heat damage or chafing against brackets.
- Step 5: Clean and inspect the sensor connector pins for corrosion before reconnecting - a loose pin can present as an open circuit intermittently.
Live Data & Freeze-Frame Tips
In DDDL, watch the coolant temp PID immediately after a cold start - a sensor reading that jumps straight to its maximum value within seconds of key-on (before the engine could physically heat up) confirms an electrical fault, not real heat.
Safety Warnings
- 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.
When to Limp In vs Tow
Not necessary - this is an electrical signal fault, not a real overheating condition, so the truck is safe to drive to a shop.
Prevention Tips for Fleets & Owner-Operators
- 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.
Related Fault Codes
SPN 110 FMI 0
Data valid but above normal, most severe - a genuine overheating condition rather than a circuit fault.
SPN 111 FMI 1
Low coolant level - a separate but related cooling system fault worth checking during the same diagnosis.
Frequently Asked Questions
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.
How do I reset it without a scanner?
Repairing the sensor or connector and cycling the key will typically clear the code without needing to manually reset anything.
Is this an expensive repair?
No - it's usually a $40-$100 sensor or a simple connector cleaning, one of the more affordable codes on this list.
Why does the cooling fan run constantly with this code?
The ECM defaults to running the fan when it can't trust the coolant temp signal, as a safety precaution against a real overheating scenario it can't rule out.