SPN 1641 FMI 0CoolingHigh
Engine Fan Speed — High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1641 / FMI 0
Category
Cooling
Fault Type
sensor
FMI Category
performance
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Fan clutch — on engine front
Common Symptoms
- Fan speed high
- Fan clutch stuck on
- Excessive fuel consumption
- Fan noise elevated
Root Causes
- Fan clutch stuck on
- Fan clutch solenoid failed
- ECM commanding fan on continuously
- Fan speed sensor fault
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Fan clutch failure common at 300k–500k miles.
System Interactions
SPN 1641 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021432 — Fan Speed High 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 stuck on causes excessive fuel consumption.
Detroit
DD15: Check fan clutch solenoid before replacement.
PACCAR
MX-13: Check ECM fan command before condemning clutch.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1641 FMI 0 — 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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