SPN 4374 FMI 5AftertreatmentMedium
DEF Dosing Valve - 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 4374 / FMI 5
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: DEF dosing valve mounted on the DEF supply module or directly on the SCR inlet mixing pipe, typically on the exhaust aftertreatment system beneath or behind the cab.
Common Symptoms
- DEF dosing valve not responding to command
Root Causes
- Open circuit in DEF dosing valve wiring harness or connector Corroded, damaged, or backed-out pins in the DEF dosing valve connector Failed or open-circuit DEF dosing valve solenoid coil Broken or chafed wiring between the ACM/DEF controller and the dosing valve Blown fuse or open relay in the DEF dosing valve supply circuit Poor ground connection at the DEF dosing valve or ACM module ACM (Aftertreatment Control Module) internal driver fault causing inability to supply current Contaminated or corroded DEF dosing valve due to urea crystallization buildup
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Open circuit — check wiring before replacing valve.
System Interactions
SPN 4374 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
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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4374 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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