SPN 3509 FMI 0Aftertreatment
Aftertreatment 1 Diesel Exhaust Fluid Tank Temperature 2 - Data Valid But Above Normal
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Overview
Fault Code
SPN 3509 / FMI 0
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: DEF tank temperature sensor 2 is located on or inside the DEF tank assembly, typically mounted on the DEF pump/sensor module on the tank, under the vehicle frame rail.
Common Symptoms
- DEF system fault lamp illuminated
- DEF quality lamp illuminated
- DEF consumption higher than normal
- DEF pump may not operate properly
- Engine derate warning or countdown timer active
- Reduced engine performance
Root Causes
- DEF tank overheating due to high ambient temperature combined with low DEF level DEF tank temperature sensor stuck high or out of calibration DEF dosing system not circulating DEF properly, causing heat buildup DEF return line restricted or blocked, preventing cooling circulation DEF tank heating element stuck ON or shorted, causing excessive heat Faulty ACM/DCU misreading sensor signal DEF tank insulation damage causing heat absorption from exhaust or engine components Contaminated DEF causing altered thermal properties
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Active DPF regeneration may be inhibited or suspended when DEF tank temperature exceeds upper threshold to prevent further thermal stress on the DEF system. Inducement warnings and torque derate may activate if fault persists alongside SCR-related faults.
System Interactions
SPN 3509 FMI 0 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 3509 FMI 0 — 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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