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Fault Code Reference

SPN 110 FMI 4ThermalHighRegen Inhibit

Engine Coolant Temperature — Voltage Below Normal or Shorted Low

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Overview

Fault Code

SPN 110 / FMI 4

Category

Thermal

Fault Type

electrical

FMI Category

voltage low

Severity

High

Derate Risk

Yes

Regen Inhibit

Yes — active regen blocked

Inducement Risk

No

Component Location: Engine coolant temperature sensor threaded into engine block, cylinder head, or coolant crossover pipe. Location varies by OEM: Cummins ISX — cylinder head coolant crossover; Detroit DD15 — front of engine block; PACCAR MX-13 — cylinder head; Volvo MP8 — coolant crossover pipe.

Common Symptoms

  • Engine derate active — ECM defaults to limp mode without valid coolant temp signal
  • Active regen inhibited — DPF regeneration disabled without valid coolant temp
  • Temperature gauge may read minimum or zero on dash
  • Cold start fueling incorrect — ECM cannot calculate proper warm-up enrichment
  • Fan may run continuously — ECM commands cooling fan on without valid temp signal
  • No overheat protection — ECM cannot detect actual overheating condition

Root Causes

  • Coolant temperature sensor shorted to ground — signal wire grounded
  • Coolant temperature sensor internal failure — sensor output stuck low
  • Coolant temperature sensor connector damage — signal pin shorted to ground pin
  • Wiring harness short to ground — signal wire contacting chassis or engine ground
  • ECM analog input circuit fault — internal ECM failure (rare)
  • Coolant temperature sensor failed open — thermistor open circuit causing low voltage reading

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Derate Behavior

Power Derate

Yes

Active Regen Inhibited

This fault prevents the DPF from completing an active regeneration cycle. DPF soot load will increase until the fault is resolved.

Mileage / Regen Notes

ECT sensor open circuit failure increases above 300,000 miles. Harness short to ground risk increases with age and heat cycling.

System Interactions

SPN 110 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: Cummins TSB 4021789 — Engine Coolant Temperature Sensor Diagnosis; Detroit TSB 13-011 — ECT Sensor Circuit Diagnosis; PACCAR TSB TH-001 — Coolant Temperature Sensor Fault Diagnosis

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

OEM Notes

Cummins

ISX/X15: ECT sensor open circuit (FMI 4) is common on high-mileage ISX engines. Replace sensor and inspect connector. INSITE ECT sensor test required.

Detroit

DD15/DD13: ECT sensor harness short to ground near exhaust manifold is a known issue. Inspect harness at exhaust area. DDDL coolant temp test required.

PACCAR

MX-13: ECT sensor signal wire short to ground is a known failure mode. Inspect connector and harness carefully. DAVIE4 ECT test required.

Volvo

MP8: ECT sensor open circuit failure is common on MP8. Replace sensor and inspect connector seal. PTT coolant temp test required.

Deep-dive diagnostic page: SPN 110 FMI 4 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.

Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.

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