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Fault Code Reference

SPN 4364 FMI 0High

SCR NOx Conversion Efficiency - Very Low

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Overview

Fault Code

SPN 4364 / FMI 0

Severity

High

Regen Inhibit

No

Inducement Risk

No

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

Common Symptoms

  • MIL lamp illuminated. DEF warning lamp on. Elevated NOx tailpipe emissions. Failed emissions testing. Ammonia smell from exhaust (overdosing compensation). Reduced fuel economy due to excessive DEF consumption. Possible white haze from tailpipe. Driver may report DEF gauge dropping faster than normal. Engine may enter inducement/derate countdown on dash display.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault does not directly inhibit DPF active regeneration. However, the ECM may delay or modify regen strategy if SCR outlet temps are abnormal. If combined with DPF-related codes, the system may force parked regen or inhibit regen entirely. Monitor for secondary DPF soot loading codes that may develop if regen is affected.

System Interactions

SPN 4364 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed universal TSBs — verify in OEM service portal for specific manufacturer. Common relevant service bulletins address: SCR catalyst replacement criteria, DEF contamination diagnosis procedures, and NOx sensor calibration updates. Check for software/calibration updates that may adjust efficiency thresholds.

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 0 — 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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