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Fault Code Reference

SPN 3594 FMI 14AftertreatmentInducement Risk

Aftertreatment 1 SCR Catalyst System - Special Instructions

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Overview

Fault Code

SPN 3594 / FMI 14

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: SCR catalyst is located in the aftertreatment assembly downstream of the DPF, typically mounted on the chassis rail or under the cab/frame on the exhaust outlet side.

Common Symptoms

  • SCR system performance warning displayed
  • DEF quality or contamination message
  • Request for SCR catalyst service or inspection
  • Aftertreatment system needs calibration or relearn
  • NOx conversion efficiency low warning
  • Check engine light or amber warning lamp illuminated

Root Causes

  • SCR catalyst efficiency below threshold requiring operator action or service intervention DEF (diesel exhaust fluid) quality issue — diluted, contaminated, or incorrect concentration DEF doser/injector malfunction causing insufficient urea delivery to SCR catalyst Accumulated SCR catalyst damage from repeated DEF quality failures NOx sensor (upstream or downstream) providing inaccurate readings affecting SCR efficiency calculation DEF supply module or pump failure reducing DEF flow ECM/ACM requiring recalibration or software update related to SCR system monitoring Catalyst brick physically damaged, melted, or degreened beyond recovery

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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 3594 FMI 14 typically indicates a special instruction or operator intervention condition related to SCR system performance. Active regen may be inhibited or limited. The system may enter a derate condition or demand a service event before normal regen and SCR operation resume.

System Interactions

SPN 3594 FMI 14 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 3594 FMI 14 — 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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