SPN 4754 FMI 31AftertreatmentInducement Risk
Aftertreatment 1 DEF Doser 1 - Condition Exists
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 4754 / FMI 31
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF doser valve is mounted directly on the DEF injection point of the exhaust aftertreatment system, typically on the decomposition tube upstream of the SCR catalyst.
Common Symptoms
- DEF system malfunction warning lamp illuminated
- Poor DEF spray pattern or atomization
- DEF consumption higher or lower than expected
- Reduced SCR efficiency or NOx conversion
- White smoke or DEF crystallization at exhaust outlet
- DEF system inducement countdown timer active
- Check engine light with aftertreatment fault
Root Causes
- DEF doser valve clogged or crystallized with urea deposits DEF doser valve stuck open or closed due to contamination Low DEF quality or contaminated DEF (wrong concentration) DEF supply line restriction or blockage upstream of doser DEF doser electrical circuit fault (open, short, or high resistance) ACM software fault or calibration mismatch requiring reflash DEF pump not delivering adequate pressure to doser DEF doser return line restricted or blocked Failed DEF doser valve (internal mechanical failure) Air in DEF lines causing inconsistent dosing
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Derate Behavior
Power Derate
Yes
SCR Inducement Risk
If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.
Mileage / Regen Notes
SPN 4754 FMI 31 indicates a general condition exists with the DEF doser. Active faults related to the DEF doser will typically inhibit or interrupt active DPF regeneration since SCR dosing and aftertreatment function are compromised. Passive regen may continue if exhaust temperatures are sufficient, but active regen and NOx reduction efficiency will be degraded. Prolonged faults may trigger derate conditions.
System Interactions
SPN 4754 FMI 31 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 4754 FMI 31 — 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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