SPN 3484 FMI 31AftertreatmentMediumRegen Inhibit
Aftertreatment Regeneration Inhibited — Special Instruction
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3484 / FMI 31
Category
Aftertreatment
Fault Type
actuator
FMI Category
protection
Severity
Medium
Derate Risk
No
Regen Inhibit
Yes — active regen blocked
Inducement Risk
No
Component Location: Regen inhibit switch — cab, dash; aftertreatment system — exhaust tunnel
Common Symptoms
- Regen inhibited
- DPF soot accumulation
- Regen lamp on
- Multiple faults may be active
Root Causes
- Multiple aftertreatment faults inhibiting regen
- Driver regen inhibit switch active
- System conditions not met for regen
- Regen inhibit fault active
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Derate Behavior
Power Derate
No
Active Regen Inhibited
This fault prevents the DPF from completing an active regeneration cycle. DPF soot load will increase until the fault is resolved.
Mileage / Regen Notes
Regen inhibit can occur at any mileage. Always indicates underlying fault.
System Interactions
SPN 3484 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
TSB Reference: Cummins TSB 4021965 — Regen Inhibit Diagnosis
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.
OEM Notes
Cummins
X15: INSITE shows regen inhibit reason.
Detroit
DD15: DDDL shows regen status and inhibit reasons.
PACCAR
MX-13: Check all active faults before attempting regen.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3484 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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