SPN 3055 FMI 31Aftertreatment
Aftertreatment 1 Diesel Particulate Filter Regeneration Inhibited Due to Active Service Regeneration
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Overview
Fault Code
SPN 3055 / FMI 31
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Aftertreatment system mounted on vehicle chassis or frame rail, typically under cab or behind cab; ACM (Aftertreatment Control Module) controls DPF regen strategy.
Common Symptoms
- DPF regeneration cannot be initiated
- Warning light or message indicating regeneration inhibited
- Inability to complete parked regeneration cycle
- Engine derate after prolonged inhibit condition
- High exhaust backpressure warning
Root Causes
- Technician or driver manually initiated a parked/active service regeneration that is currently in progress DPF soot load level high enough to require a forced active regen, inhibiting automatic passive regen Passive regen conditions not being met due to low exhaust temperatures from short-cycle or low-load operation DPF delta pressure sensor reading elevated soot load triggering ACM to command active service regen Previous incomplete regen cycle left DPF partially loaded, requiring a new forced active regen Repeated regen inhibit events (e.g., operator pressing inhibit switch) causing soot accumulation requiring active service regen Faulty DPF differential pressure sensor providing false high soot load readings causing unnecessary active regen commands
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 3055 FMI 31 is a status/informational fault indicating that DPF regeneration (passive or automatic active) is inhibited because an active service regeneration is currently in progress. This is expected behavior during a technician-commanded or system-commanded parked regen. Normal automatic regen resumes after the active service regen cycle completes successfully. If active service regen is blocked from completing, DPF soot load will continue to rise and may result in derate or engine protection shutdown.
System Interactions
SPN 3055 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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3055 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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