SPN 110 FMI 3 — Engine Coolant Temperature Circuit Voltage Above Normal
ECT high-voltage codes are open-circuit diagnostics before cooling hardware conclusions. Thermostat replaced while the ECT sensor circuit was open is the most documented misdiagnosis.
Fault Overview
SPN 110 FMI 3 indicates the engine coolant temperature sensor circuit voltage is above the normal maximum — an open circuit or high-signal class fault. Open signal wire, failed thermistor, unplugged connector after thermostat or housing service, corroded terminals, and reference or ground issues are the primary causes. An open ECT circuit typically reports an extremely cold value — fan and derate logic can be confused by false cold ECT data.
Engine & Platform Coverage
Possible Causes
- 1.Open signal wire from damage near thermostat housing or head
- 2.Failed ECT thermistor with internal open element
- 3.Unplugged connector after thermostat or housing service
- 4.Corroded terminals at ECT sensor connector
- 5.Reference or ground anomaly affecting sensor output
Wiring & Circuit Notes
Inspect the ECT sensor connector at the thermostat housing or head for chafe near brackets and coolant contamination of pins. Confirm connector is fully locked after any recent thermostat or housing service. Verify 5V reference and ground at the sensor connector.
DVOM Specs & Sensor Tests
Confirm 5V reference, ground, and signal at the ECT sensor connector. An open circuit often reads extremely cold while the engine is at operating temperature — IR correlation confirms the true coolant temperature. Cold-soak should approximate ambient once the circuit is healthy. Replace the sensor after circuit is proven. Evaluate fan and derate behavior only with valid ECT data.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Confirm connector is fully locked after any recent thermostat or housing service
- 2Verify 5V reference and ground at ECT sensor connector
- 3Check signal wire continuity from sensor to ECM
- 4IR-correlate true coolant temperature versus ECT PID value
- 5Replace ECT sensor after circuit is proven clean
- 6Evaluate fan and derate behavior only with valid ECT data
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Confirm connector is fully locked after any recent thermostat or housing service
- 2Verify 5V reference and ground at ECT sensor connector
- 3Check signal wire continuity from sensor to ECM
- 4IR-correlate true coolant temperature versus ECT PID value
- 5Replace ECT sensor after circuit is proven clean
- 6Evaluate fan and derate behavior only with valid ECT data
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
Related Resources
Related Systems
Related Fault Codes
Related Sensor Locations
Common Symptoms
Related Articles
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
SPN 110 FMI 3 is an open-circuit ECT diagnosis. The sensor circuit must be proven valid before any cooling-system hardware conclusions.