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Fault Code Reference

SPN 4364 FMI 1High

SCR NOx Conversion Efficiency - Very Low (Alt)

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Overview

Fault Code

SPN 4364 / FMI 1

Severity

High

Regen Inhibit

No

Inducement Risk

No

Component Location: SCR catalyst assembly is located in the aftertreatment system downstream of the DPF, typically mounted on the frame rail or under the cab/sleeper area of the vehicle.

Common Symptoms

  • Active fault code with MIL illuminated
  • Excessive tailpipe NOx emissions exceeding threshold
  • Failed emissions inspection or OBD readiness monitor
  • Reduced DEF consumption noted over time
  • SCR system not achieving expected NOx reduction efficiency
  • Possible white exhaust smoke if urea dosing is excessive

Root Causes

  • Degraded or contaminated SCR catalyst (aged or heat-damaged substrate)
  • Incorrect or contaminated DEF fluid (diluted, wrong concentration, or non-AUS32)
  • Faulty or clogged DEF dosing injector causing inadequate urea injection
  • DEF doser pump failure or low DEF pressure reducing dosing quantity
  • NOx sensor (downstream) failure giving false low-efficiency reading
  • NOx sensor (upstream) failure skewing conversion efficiency calculation
  • Exhaust temperature too low for SCR catalyst activation (below 200°C)
  • DPF ash loading causing excessive backpressure and reduced SCR effectiveness
  • DEF supply line restriction or frozen DEF in cold weather
  • SCR catalyst substrate cracked or mechanically damaged

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Derate Behavior

Mileage / Regen Notes

Catalyst damage possible

System Interactions

SPN 4364 FMI 1 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: SCR catalyst TSB

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.

Deep-dive diagnostic page: SPN 4364 FMI 1 — 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.

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

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