SPN 1643 FMI 31CoolingMedium
Engine Coolant Fan Control — Special Instruction
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1643 / FMI 31
Category
Cooling
Fault Type
actuator
FMI Category
protection
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Fan control system — ECM, fan clutch, coolant temperature sensor
Common Symptoms
- Fan control fault
- Fan operating in default mode
- Cooling system monitoring affected
Root Causes
- Fan control system fault
- Multiple cooling system faults
- ECM fan control fault
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Fan control faults can occur at any mileage.
System Interactions
SPN 1643 FMI 31 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021434 — Fan Control System 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: Fan control fault may cause fan to run continuously.
Detroit
DD15: DDDL shows fan control status.
PACCAR
MX-13: Check all active faults before addressing fan control.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1643 FMI 31 — 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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