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Fault Code Reference

SPN 3609 FMI 3AftertreatmentInducement Risk

DEF Pump Motor - Voltage Above Normal or Shorted High

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Overview

Fault Code

SPN 3609 / FMI 3

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF pump motor is typically mounted on or near the DEF tank, often on the driver-side frame rail or beneath the cab in the aftertreatment module assembly.

Common Symptoms

  • DEF pump motor inoperative or intermittent operation
  • Active fault lamp illuminated (MIL or SCR warning)
  • DEF dosing interrupted or suspended
  • Elevated NOx emissions
  • SCR system derating or derate warning active
  • Failed stationary or parked regen due to DEF system fault

Root Causes

  • Short to voltage (B+) in DEF pump motor control wiring harness
  • Failed DEF pump motor with internal short causing high voltage feedback
  • Chafed or damaged wiring harness shorting to power rail
  • Faulty ACM/DCU output driver internally shorted high
  • Corroded or moisture-contaminated connector at DEF pump motor
  • Failed DEF pump relay providing unswitched voltage to motor circuit
  • Incorrect aftermarket wiring or splice causing short to power

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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

SPN 3609 FMI 3 will inhibit DEF dosing, preventing SCR catalyst from reducing NOx. Active regen may be blocked or incomplete. Continued operation will likely trigger a derate (5 mph or torque reduction) after fault persistence thresholds are met per OEM SCR fault escalation logic.

System Interactions

SPN 3609 FMI 3 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 3609 FMI 3 — 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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