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Fault Code Reference

SPN 641 FMI 6Medium

Fan Speed - Current Above Normal

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Overview

Fault Code

SPN 641 / FMI 6

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: Fan control solenoid is typically mounted on the front of the engine near the cooling fan hub or fan clutch assembly, accessible from the front of the engine compartment.

Common Symptoms

  • Cooling fan runs at full speed constantly regardless of engine temperature. Fan may cycle erratically or surge unexpectedly. Excessive fan noise at idle or low load conditions. Possible overcooling issues in cold weather. Check engine light illuminated. Fan clutch or fan drive may feel hot to the touch. Increased fuel consumption due to parasitic load from fan running continuously.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault typically does not directly inhibit active DPF regeneration. However, if the fan is stuck running at full speed, it may cause difficulty achieving and maintaining proper regen temperatures, potentially leading to incomplete regens or extended regen times. Monitor EGT temps during regen if this code is active.

System Interactions

SPN 641 FMI 6 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 engines have known harness chafe points near fan clutch documented in service bulletins. Detroit DD13/DD15 have service information regarding fan clutch connector sealing. Check OEM-specific service information system for latest updates.

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 6 — 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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