SPN 110 FMI 4 — Engine Coolant Temperature Circuit Voltage Below Normal
ECT low-voltage codes need short isolation before cooling-system conclusions. False overheat derates and unnecessary thermostat or water-pump work from shorted ECT circuits is the most documented misdiagnosis.
Fault Overview
SPN 110 FMI 4 indicates the engine coolant temperature sensor circuit voltage is below the normal minimum — a short circuit or low-signal class fault. Signal short to ground, failed sensor with internal short, crushed harness near thermostat housing, moisture bridging, and insulation damage are the primary causes. A shorted ECT circuit typically reports an extremely hot value — false overheat derates and unnecessary cooling-system work are the most documented consequences.
Engine & Platform Coverage
Possible Causes
- 1.Signal wire short to ground at pinch points near thermostat housing
- 2.Failed ECT sensor with internal short to ground
- 3.Crushed harness near thermostat housing or water-pump area
- 4.Moisture bridging at connector body near coolant system
- 5.Insulation damage at harness clips near thermostat housing
Wiring & Circuit Notes
Inspect pinch points near the thermostat housing and water-pump area — these are the highest-probability short locations for the ECT circuit. Check for moisture in the connector body. Disconnect isolation identifies whether the short is in the sensor or the harness.
DVOM Specs & Sensor Tests
Disconnect the ECT sensor and check the signal wire for short to ground. If the short clears with the sensor disconnected, the sensor is internally shorted. If the short persists, inspect the harness at thermostat housing and water-pump area pinch points. After repair, IR-correlate true coolant temperature versus ECT PID to confirm the repaired sensor is tracking plausibly. Evaluate true overheat only if temperature is real after circuit is proven.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Disconnect ECT sensor connector
- 2Check signal wire for short to ground with sensor disconnected
- 3If short clears with sensor disconnected, replace ECT sensor
- 4If short persists, inspect harness at thermostat housing and water-pump area pinch points
- 5Repair harness short at identified location
- 6IR-correlate true coolant temperature after repair to confirm plausible tracking
- 7Evaluate true overheat only if temperature is real after circuit is proven
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
5 steps locked- 1Disconnect ECT sensor connector
- 2Check signal wire for short to ground with sensor disconnected
- 3If short clears with sensor disconnected, replace ECT sensor
- 4If short persists, inspect harness at thermostat housing and water-pump area pinch points
- 5Repair harness short at identified location
- 6IR-correlate true coolant temperature after repair to confirm plausible tracking
- 7Evaluate true overheat only if temperature is real after circuit is proven
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
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Summary
SPN 110 FMI 4 requires short isolation before any cooling-system conclusions. Prove the ECT circuit first — true overheat evaluation comes only after the circuit is clean.