SPN 110 FMI 1CoolingMediumRegen Inhibit
Engine Coolant Temperature — Low
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 110 / FMI 1
Category
Cooling
Fault Type
sensor
FMI Category
performance
Severity
Medium
Derate Risk
No
Regen Inhibit
Yes — active regen blocked
Inducement Risk
No
Component Location: Thermostat — in thermostat housing on engine; coolant temperature sensor — on thermostat housing
Common Symptoms
- Engine not reaching operating temperature
- Coolant temperature low
- Regen inhibited
- Fuel economy reduced
- White smoke
Root Causes
- Thermostat stuck open
- Coolant temperature sensor fault
- Engine operating in extreme cold
- Thermostat failed
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Derate Behavior
Power Derate
No
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
Thermostat failure common at 200k–400k miles.
System Interactions
SPN 110 FMI 1 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021403 — Engine Coolant Temperature Low 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
X15: Thermostat stuck open is most common cause.
Detroit
DD15: Check thermostat operation during warm-up.
PACCAR
MX-13: Check coolant temperature sensor accuracy.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 110 FMI 1 — 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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