SPN 4364 FMI 5AftertreatmentMedium
SCR NOx Efficiency Sensor - Current Below Normal
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 4364 / FMI 5
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: Downstream NOx sensor is mounted on the exhaust outlet pipe exiting the SCR catalyst, typically located under the cab or mid-chassis on the aftertreatment assembly.
Common Symptoms
- Poor SCR performance reading, check engine light
Root Causes
- Open circuit in downstream NOx sensor signal or power wiring Faulty downstream NOx sensor (failed open internally) Corroded or damaged connector at the downstream NOx sensor Broken or chafed wiring harness between NOx sensor and aftertreatment control module Failed NOx sensor heater circuit causing low current condition Aftertreatment Control Module (ACM) output driver failure Incorrect or degraded DEF quality reducing SCR efficiency below sensor threshold Plugged or failed DEF doser injector affecting SCR performance
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Sensor open circuit — check wiring before replacing.
System Interactions
SPN 4364 FMI 5 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
TSB Reference: Efficiency sensor 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.
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4364 FMI 5 — 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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