SPN 3243 FMI 5AftertreatmentInducement Risk
Aftertreatment Diesel Exhaust Fluid Tank Temperature 2 - Current Below Normal
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Overview
Fault Code
SPN 3243 / FMI 5
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF tank temperature sensor 2 is mounted on or inside the DEF tank assembly, typically on the underside or side of the tank near the rear of the vehicle frame.
Common Symptoms
- DEF system fault lamp illuminated
- Engine derate or power reduction
- DEF system performance warning message
- Reduced DEF injection efficiency
- Increased NOx emissions during regeneration
Root Causes
- Open circuit in DEF tank temperature sensor 2 signal or power wire Corroded or damaged connector at DEF tank temperature sensor 2 Failed DEF tank temperature sensor 2 (open element) Broken or chafed wiring harness between sensor and ACM Poor or missing ground at sensor or ACM ground circuit Faulty ACM module (rare, only after all wiring/sensor verified)
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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 3243 FMI 5 may inhibit or delay DEF dosing and active DPF regeneration if the ACM cannot confirm DEF tank temperature is within safe operating range. Prolonged faults may trigger derate conditions and prevent regen initiation.
System Interactions
SPN 3243 FMI 5 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 3243 FMI 5 — 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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