SPN 1080 FMI 3Cooling System
Engine Fan Speed - Voltage Above Normal or Shorted High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1080 / FMI 3
Category
Cooling System
Derate Risk
Possible
Regen Inhibit
No
Inducement Risk
No
Component Location: Fan speed sensor is typically mounted on the fan hub or fan shroud assembly near the engine cooling fan.
Common Symptoms
- Engine fan runs at maximum speed constantly
- High voltage DTC for fan speed sensor
- Fan speed reading shows erratic or maximum value on scan tool
- Fan clutch may not modulate properly
- Cooling system operates in fail-safe mode
Root Causes
- Shorted fan speed sensor wiring or connector Faulty fan speed sensor Failed engine cooling fan clutch control module Damaged wiring harness causing short to voltage Faulty ECM sensing circuit
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Derate Behavior
Power Derate
Possible
System Interactions
SPN 1080 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
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OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1080 FMI 3 — 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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