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Fault Code Reference

SPN 3685 FMI 4AftertreatmentInducement Risk

Aftertreatment 1 DEF Dosing Unit Pump Motor - Voltage Below Normal or Shorted Low

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Overview

Fault Code

SPN 3685 / FMI 4

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF dosing unit pump motor is typically located on the DEF pump module mounted near the DEF tank or frame rail, connected via harness routed to the ACM.

Common Symptoms

  • Check engine light illuminated
  • DEF system malfunction indicator active
  • Engine derate or power reduction
  • DEF dosing inoperative
  • Reduced SCR efficiency
  • Possible limp mode activation

Root Causes

  • DEF doser pump motor internal short to ground causing voltage below normal Chafed or damaged wiring harness shorting DEF pump motor circuit to ground or chassis Corroded or water-intruded connector at DEF dosing unit pump motor Failed DEF dosing unit pump motor drawing excessive current Damaged ACM output driver circuit for DEF pump motor Open or high-resistance ground circuit in DEF pump motor circuit DEF pump motor power supply relay failure causing low voltage condition Pinched wiring near DEF pump mounting bracket or frame rail

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

An active SPN 3685 FMI 4 will inhibit DEF dosing to the SCR catalyst, causing SCR system deactivation. This will result in inhibited or disabled DPF active regeneration in severe cases, and the vehicle may enter a derate condition with reduced NOx conversion efficiency. Continued operation without DEF dosing may trigger SCR-related derates and eventual engine torque reduction or speed limiting per Cummins aftertreatment protection strategy.

System Interactions

SPN 3685 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components

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.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 3685 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.

EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx 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.

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