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Fault Code Reference

SPN 641 FMI 4Medium

Fan Speed - Voltage Low

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Overview

Fault Code

SPN 641 / FMI 4

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: Fan speed sensor is typically mounted on the fan clutch hub or fan shroud assembly at the front of the engine.

Common Symptoms

  • Engine cooling fan not engaging or running at full speed constantly; engine overheating or running hotter than normal; A/C performance reduced due to lack of condenser airflow; fan speed parameter reading zero or erratic on scan tool; audible fan clutch cycling issues; MIL or warning lamp illuminated

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault typically does not directly inhibit active DPF regeneration. However, if the ECM cannot verify fan speed, it may limit regen capability on some platforms as a protective measure to prevent overheating. On most engines, regen will proceed but the ECM may command fan ON as a default safety strategy. Verify regen inhibit status with scan tool if regen issues are present alongside this code.

System Interactions

SPN 641 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins ISX and Detroit DD13/DD15 have known harness routing issues near fan shroud causing chafing. Check OEM-specific bulletins for fan speed sensor connector upgrades and harness re-route instructions.

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.

Deep-dive diagnostic page: SPN 641 FMI 4 — 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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