SPN 647 FMI 2Engine Protection
Fan Speed - Erratic/Intermittent
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 647 / FMI 2
Category
Engine Protection
Derate Risk
Possible
Regen Inhibit
No
Inducement Risk
No
Component Location: Fan speed sensor is typically mounted on the fan shroud or fan hub assembly near the engine cooling fan.
Common Symptoms
- Intermittent fan operation or cycling
- Fan runs at full speed constantly
- Fan does not respond to temperature changes
- Engine overheat warning
- Check engine light illuminated
- Fan speed fluctuates abnormally
Root Causes
- Faulty fan speed sensor or damaged sensor wiring Intermittent connection or corrosion at fan speed sensor connector Damaged fan clutch or fan hub assembly causing erratic operation ECM supply voltage issue to fan speed sensor circuit Mechanical fan bearing wear or debris interference
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Derate Behavior
Power Derate
Possible
System Interactions
SPN 647 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
ECM communication — J1939 network faults can affect all connected modules
Engine protection — ECM may trigger derate to prevent damage
Aftertreatment — engine protection derates can affect regen strategy
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 647 FMI 2 — 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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