SPN 4374 FMI 3AftertreatmentMedium
DEF Dosing Valve - Voltage High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 4374 / FMI 3
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: DEF dosing valve is mounted on the decomposition tube or mixer assembly upstream of the SCR catalyst in the exhaust system.
Common Symptoms
- Check engine light illuminated. DEF warning lamp may be on. No DEF injection or erratic dosing. Possible ammonia smell from exhaust (over-dosing before fault set). Low NOx conversion efficiency. SCR inlet NOx and outlet NOx readings may be similar on scan tool. Possible white smoke or DEF crystallization at tailpipe if valve was stuck open.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Active regeneration is typically NOT inhibited by this fault alone — DPF regen operates independently of DEF dosing. However, if the SCR system cannot dose properly, NOx conversion will suffer and the system may set additional SCR efficiency faults. Prolonged operation without proper dosing can lead to SCR catalyst damage and eventual inducement. Check for companion codes related to SCR efficiency or NOx sensor rationality.
System Interactions
SPN 4374 FMI 3 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: No confirmed universal TSBs — verify in OEM service portal. Cummins has issued service letters on DEF dosing harness routing near exhaust heat shields. Detroit Diesel has bulletins on aftertreatment harness connector sealing. Freightliner Cascadia models may have harness chafing bulletins for chassis-routed DEF wiring. Always check OEM-specific portal for current TSB applicability.
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
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4374 FMI 3 — 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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