SPN 3249 FMI 2Aftertreatment
Aftertreatment 1 Fuel Pressure 1 - Data Erratic
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Overview
Fault Code
SPN 3249 / FMI 2
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The aftertreatment fuel pressure sensor is typically mounted on the dosing fuel supply line or fuel rail near the decomposition reactor or DPF inlet.
Common Symptoms
- Active DPF regeneration fails to complete
- Check engine light illuminated
- Diesel exhaust fluid warning lamp may activate
- Intermittent engine derate during regeneration attempts
- Excessive fuel consumption during regeneration cycles
Root Causes
- Failing or contaminated aftertreatment fuel pressure sensor Air intrusion or aeration in the aftertreatment fuel supply line causing erratic pressure signal Clogged or restricted aftertreatment fuel injector (doser injector) affecting system pressure Aftertreatment fuel supply pump (HC doser pump) weak or failing, producing unstable pressure Wiring harness chafing or intermittent connection at the fuel pressure sensor connector Fuel supply line restriction or partial blockage upstream of the dosing module ACM software or calibration issue causing erratic data interpretation Contaminated or degraded diesel fuel in the aftertreatment fuel supply circuit
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
An erratic aftertreatment fuel pressure signal (FMI 2) will typically inhibit active DPF regeneration as the ACM cannot confirm safe and accurate fuel delivery for the burner/doser. Parked and in-process regens may be suspended. Soot load will continue to accumulate, potentially leading to DPF over-temperature or high soot load faults if unresolved.
System Interactions
SPN 3249 FMI 2 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 3249 FMI 2 — 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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