SPN 4364 FMI 4AftertreatmentMedium
SCR NOx Efficiency Sensor - Voltage Low
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Overview
Fault Code
SPN 4364 / FMI 4
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: Downstream NOx sensor is mounted on the SCR outlet exhaust pipe, typically located after the SCR catalyst brick near the rear undercarriage of the vehicle.
Common Symptoms
- Active fault code for SCR NOx efficiency sensor voltage low
- MIL or CEL illuminated
- DEF/SCR system warning lamp active
- Possible derate or engine power reduction
- Failed or inaccurate NOx conversion efficiency readings
- Possible inhibited or forced DPF regeneration
Root Causes
- Faulty or failed downstream NOx sensor (open or short to ground internally)
- Damaged, corroded, or loose wiring harness connector at NOx sensor
- Chafed or shorted wiring between NOx sensor and aftertreatment control module
- Poor ground connection at NOx sensor or aftertreatment module
- Failed NOx sensor heater causing low signal voltage
- Aftertreatment control module (ACM/DCU) supply voltage issue
- Failed aftertreatment control module (rare)
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Check wiring before replacing sensor.
System Interactions
SPN 4364 FMI 4 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 4 — 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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