SPN 3515 FMI 16AftertreatmentInducement Risk
Aftertreatment 1 DEF Dosing Unit Temperature - Data Valid But Above Normal Moderately Severe
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Overview
Fault Code
SPN 3515 / FMI 16
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF dosing unit (injector) is mounted on the DEF delivery pipe upstream of the SCR catalyst inlet cone, typically on the mid-to-rear underside of the vehicle chassis near the aftertreatment assembly.
Common Symptoms
- DEF dosing unit high temperature warning lamp illuminated
- Engine power derate or limited performance
- DEF consumption appears reduced or stopped
- Aftertreatment system efficiency reduced
- Check engine light active
Root Causes
- DEF dosing unit (injector) overheating due to insufficient DEF flow or DEF line restriction Failed or stuck-closed DEF supply pump not delivering adequate cooling flow through dosing unit DEF line blockage or crystallization restricting flow and causing thermal buildup at injector DEF dosing unit internal failure causing excess heat generation Faulty DEF dosing unit temperature sensor reporting falsely elevated temperature Exhaust backflow or high exhaust backpressure forcing hot gases toward dosing unit DEF tank level critically low causing pump cavitation and loss of cooling flow DEF quality issue (diluted or contaminated DEF) affecting thermal properties and pump performance ACM software fault or miscalibration causing incorrect temperature interpretation
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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 3515 FMI 16 (DEF dosing unit temperature moderately severe above normal) will typically inhibit active DPF regeneration and disable DEF dosing to protect the injector from thermal damage. The ACM may command a DEF purge cycle and suspend dosing until temperature returns to safe range. Continued operation with fault active risks SCR derating, NOx non-compliance, and eventual SCR inducement/torque derate depending on OEM escalation thresholds.
System Interactions
SPN 3515 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3515 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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