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Fault Code Reference

SPN 3301 FMI 18AftertreatmentInducement Risk

Aftertreatment 1 SCR Catalyst Conversion Efficiency - Data Valid But Below Normal

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Overview

Fault Code

SPN 3301 / FMI 18

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: SCR catalyst assembly is located downstream of the DPF in the aftertreatment system, typically mounted on the vehicle frame rail or integrated in the exhaust aftertreatment unit (EATS) under or behind the cab.

Common Symptoms

  • DEF consumption higher than normal
  • NOx sensor readings show elevated NOx levels downstream of SCR
  • Check engine light illuminated
  • Low SCR conversion efficiency displayed in diagnostics
  • Failed emissions testing
  • Possible white smoke from exhaust under load

Root Causes

  • Degraded or aged SCR catalyst substrate with reduced NOx conversion efficiency
  • DEF quality issue — diluted, contaminated, or incorrect concentration (not 32.5% urea)
  • DEF doser injector clogging or improper spray pattern reducing reagent atomization
  • Insufficient DEF dosing due to faulty DEF pump or dosing valve
  • Exhaust temperature too low for proper SCR catalyst activation (below ~200°C)
  • DPF soot or ash loading affecting exhaust flow and SCR inlet conditions
  • NOx sensor (downstream) failure or drift providing false low-efficiency reading
  • NOx sensor (upstream) failure causing incorrect dosing calculation
  • DEF lines kinked, frozen, or internally blocked reducing flow to doser
  • SCR catalyst physical damage — cracking, melting, or delamination from thermal event
  • Exhaust leaks upstream of SCR catalyst diluting reagent concentration
  • ACM software calibration mismatch or outdated firmware

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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.

System Interactions

SPN 3301 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 3301 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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