SPN 3491 FMI 0AftertreatmentHighInducement Risk
Aftertreatment 1 Diesel Exhaust Fluid Temperature — High
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3491 / FMI 0
Category
Aftertreatment
Fault Type
sensor
FMI Category
performance
Severity
High
Derate Risk
No
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF temperature sensor — in DEF tank
Common Symptoms
- DEF temperature high
- DEF degradation risk
- DEF quality fault may follow
- Possible inducement warning
Root Causes
- DEF tank overheating from exhaust heat
- DEF heater stuck on
- DEF temperature sensor fault
- Ambient temperature extremely high
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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
DEF temperature issues more common in hot climates.
System Interactions
SPN 3491 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
TSB Reference: Cummins TSB 4021972 — DEF Temperature High Diagnosis
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.
OEM Notes
Cummins
X15: DEF degrades above 95°F. Check tank location.
Detroit
DD15: Check DEF heater relay for stuck-on condition.
PACCAR
MX-13: DEF tank should not be adjacent to exhaust.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3491 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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