SPN 4374 FMI 4AftertreatmentMedium
DEF Dosing Valve - Voltage Low
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Overview
Fault Code
SPN 4374 / FMI 4
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 SCR inlet pipe in the exhaust aftertreatment system.
Common Symptoms
- Check engine light illuminated. DEF warning lamp on. No DEF injection occurring. Possible white smoke or ammonia smell at tailpipe due to incomplete SCR reaction. DEF fluid consumption drops to zero. SCR inlet NOx readings remain high. Possible inducement warning message on dash after extended operation.
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does not directly inhibit DPF active regeneration, as regen is controlled by diesel fuel dosing (7th injector or in-cylinder), not DEF injection. However, if the ACM enters a protective mode or multiple aftertreatment faults are present simultaneously, regen may be suspended until DEF system faults are resolved. SCR efficiency will be degraded, leading to elevated tailpipe NOx regardless of regen status.
System Interactions
SPN 4374 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: No confirmed universal TSBs — verify in OEM service portal. Cummins has issued service letters regarding DEF dosing valve harness routing and heat shield requirements. Detroit Diesel has addressed connector sealing on certain DD13/DD15 applications. Paccar has released bulletins on MX-13 DEF doser wiring concerns. Always check for model-year-specific TSBs before replacing components.
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 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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