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Fault Code Reference

SPN 3464 FMI 18AftertreatmentInducement Risk

SCR Catalyst Conversion Efficiency - Data Valid But Below Normal

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Overview

Fault Code

SPN 3464 / FMI 18

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 vehicle frame rail or integrated into the exhaust system under the cab or chassis.

Common Symptoms

  • MIL or Check Engine Light illuminated
  • DEF consumption higher than normal
  • Reduced NOx conversion efficiency reported
  • Ammonia slip detected at tailpipe
  • Poor fuel economy
  • Exhaust odor (ammonia smell)

Root Causes

  • Degraded or aged SCR catalyst brick with reduced NOx conversion efficiency
  • DEF (diesel exhaust fluid) quality issue — diluted, contaminated, or incorrect concentration
  • DEF doser/injector clogging or weak spray pattern causing poor urea distribution
  • Faulty or drifted NOx sensor (downstream) reporting incorrect low conversion readings
  • DEF supply system issue — low DEF pump pressure or restricted DEF lines reducing dosing quantity
  • SCR catalyst poisoned by sulfur, hydrocarbons, or oil contamination
  • Incorrect DEF dosing due to faulty ACM calibration or software
  • Upstream NOx sensor drift causing inaccurate inlet NOx baseline for efficiency calculation
  • Exhaust temperature too low for proper SCR catalyst activation (below light-off temp)
  • DPF excessive soot loading causing backpressure affecting NOx reduction efficiency

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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 3464 FMI 18 may inhibit or restrict active DPF regeneration if the ACM determines SCR efficiency is insufficient to safely manage NOx during regen; repeated or persistent faults may trigger reduced engine power (derate) and illuminate the MIL or amber warning lamp. Passive regen may still occur but active regen may be suppressed until fault is resolved.

System Interactions

SPN 3464 FMI 18 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 3464 FMI 18 — 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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