Mack SPN 110 FMI 3: Engine Coolant Temperature Diagnosis
SPN 110 FMI 3 (FMI 3) — Engine Coolant Temperature 1 — Voltage Above Normal or Shorted to High Source
Quick Answer
On Mack, SPN 110 FMI 3 is a Engine Coolant Temperature fault (Voltage Above Normal or Shorted to High Source). Prove wiring/live data first; do not parts-cannon the component.
Overview: Mack SPN 110 FMI 3
On Mack, SPN 110 FMI 3 is a Engine Coolant Temperature fault (Voltage Above Normal or Shorted to High Source). Prove wiring/live data first; do not parts-cannon the component. This guide covers symptoms, root causes, live-data checks, and misdiagnoses for Mack platforms.
Repair Snapshot
Time
1 - 3 Hours
Difficulty
Intermediate
Parts Cost
$50 - $800
Dealer Labor
$350+
Tools Required
- Premium Tech Tool
- Noregon JPRO
- Multimeter
What SPN 110 FMI 3 Means
SPN 110 FMI 3 on Mack means Engine Coolant Temperature 1 — Voltage Above Normal or Shorted to High Source. Confirm Compare ECT to infrared at thermostat housing with Premium Tech Tool / Noregon JPRO before replacing the engine coolant temperature.
Symptoms Drivers Notice
- Check-engine / amber warning lamp illuminated.
- Soft power, unstable idle, or poor response when engine coolant temperature data is irrational.
- Live data for engine coolant temperature fails rationality vs mechanical gauge or partner sensors.
- Codes may return immediately after clearing if root cause remains.
Most Likely Root Causes
Common Misdiagnoses (Don't Waste Parts)
Why it happens:
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Why it happens:
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Why it happens:
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Master Tech Commentary
Mechanic's Notes
Shop tip (Mack): For SPN 110 FMI 3, prove Compare ECT to infrared at thermostat housing before quoting the Engine Coolant Temperature. Common false path is replacing the sensor when the harness or reference circuit is the failure.
Step-by-Step Diagnostic Procedure
- Step 1: Connect Premium Tech Tool or Noregon JPRO and record SPN 110 FMI 3 with freeze-frame load/temp.
- Step 2: Inspect harness and connector at the engine coolant temperature for chafe, corrosion, and bent pins.
- Step 3: Verify 5V reference / ground / signal integrity with a multimeter where applicable.
- Step 4: Compare ECT to infrared at thermostat housing
- Step 5: Repair the proven fault, clear only after repair, and verify under the same conditions as freeze-frame.
Live Data & Freeze-Frame Tips
Log Engine Coolant Temperature with Premium Tech Tool under freeze-frame conditions. Compare ECT to infrared at thermostat housing. Compare to a mechanical gauge when pressure/temperature related.
When to Limp In vs Tow
Drive carefully to a shop if lamps are warning-only and power remains controllable; tow if power loss is severe.
Related Fault Codes
Frequently Asked Questions
What does Mack SPN 110 FMI 3 mean?
It indicates a Engine Coolant Temperature fault: Voltage Above Normal or Shorted to High Source. Confirm with live data and wiring tests before parts replacement.
Can SPN 110 cause a derate?
It can contribute to limp behavior if the ECM loses trust in critical data, especially when paired with other faults.
What tools diagnose this best?
Premium Tech Tool and Noregon JPRO, plus a quality multimeter and mechanical gauges for pressure/temperature checks.
Should I clear the code first?
No. Capture freeze-frame and the full stack first. Clear only after the repair and verification.
More Mack codes on this system
- Mack SPN 111 FMI 17 — Engine Coolant Level Sensor
- Mack SPN 111 FMI 9 — Coolant Level
- Mack SPN 5246 FMI 0 — SCR Operator Inducement
- Mack SPN 3216 FMI 9 — Throttle Position Sensor
- Mack SPN 4094 FMI 16 — DPF Soot Load
- Mack SPN 190 FMI 0 — Engine Speed Sensor Circuit
- Mack SPN 102 FMI 4 — Boost Pressure Sensor Circuit
- Mack SPN 5394 FMI 3 — DEF Tank Heater Circuit
Guides that go deeper
- SPN 1569 FMI 31: Engine Protection Shutdown and Safe Recovery Steps [2026 Diagnostic Guide]
SPN 1569 FMI 31 means engine protection shutdown is active—not the root fault. Read stacked codes, verify oil and coolant, then reset only after the trigger is fixed.
- Mack MP8 Aftertreatment Faults: First Checks That Save Shop Time
Mack MP8 aftertreatment stacks look noisy until you separate DPF, DEF dosing, and NOx sensor rationality. Start with these high-yield checks.
- SPN 3031 DEF Tank Temperature Fault: Heater, Sensor, or Wiring
SPN 3031 points at DEF tank temperature strategy faults. Confirm heaters, sensor rationality, and harness chafe before replacing the tank assembly.