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Fault Code Reference

SPN 641 FMI 2Medium

Fan Speed - Data Erratic

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Overview

Fault Code

SPN 641 / FMI 2

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • Erratic or fluctuating fan speed readings on scan tool; fan cycling on and off unexpectedly; fan running at full speed continuously; fan not responding to commanded speed; intermittent MIL illumination; possible overheating concerns if fan control is affected; noisy fan operation due to inconsistent speed commands; customer complaints of fan 'hunting' or surging

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 accurately determine fan speed, it may limit regen capability as a protective measure to prevent overheating. In some applications, the ECM may force the fan to run at maximum speed during regen as a failsafe when fan speed data is erratic. Monitor for related cooling system faults that could compound regen issues.

System Interactions

SPN 641 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed TSBs — verify in OEM service portal. Check for manufacturer-specific bulletins related to fan speed sensor harness routing, connector upgrades, or ECM calibration updates affecting fan control logic.

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 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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