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Fault Code Reference

SPN 3249 FMI 16AftertreatmentInducement Risk

Aftertreatment 1 Diesel Exhaust Fluid Dosing Absolute Pressure - Data Valid But Above Normal

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Overview

Fault Code

SPN 3249 / FMI 16

Category

Aftertreatment

Derate Risk

No

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: The DEF dosing unit is mounted on the aftertreatment system housing, typically near the decomposition reactor inlet.

Common Symptoms

  • DEF dosing malfunction warning lamp illuminated
  • Poor NOx conversion efficiency
  • DEF system performance degraded message
  • Higher than normal DEF consumption
  • Ammonia slip detected
  • Inducement countdown active

Root Causes

  • DEF dosing unit (pump/injector assembly) internal pressure relief valve stuck closed or restricted Blocked or kinked DEF supply line causing pressure buildup DEF dosing injector tip clogged or partially restricted causing back pressure DEF filter clogged or restricted increasing system pressure Faulty DEF pressure sensor reporting falsely high pressure DEF pump running above commanded duty cycle due to pump failure Air in DEF lines causing erratic pressure spikes DEF return line restricted or kinked

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Derate Behavior

Power Derate

No

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 3249 FMI 16 will typically inhibit or interrupt active DPF regeneration due to unreliable DEF dosing pressure; SCR dosing may be suspended, causing NOx conversion loss and potential SCR efficiency fault escalation.

System Interactions

SPN 3249 FMI 16 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.

Deep-dive diagnostic page: SPN 3249 FMI 16 — 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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