SPN 3243 FMI 4Aftertreatment
Aftertreatment DEF Tank Temperature 2 - Voltage Below Normal or Shorted Low
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Overview
Fault Code
SPN 3243 / FMI 4
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: The DEF tank temperature sensor is located in or on the DEF tank, typically mounted in the tank housing.
Common Symptoms
- DEF tank temperature sensor 2 reading implausibly low or shorted to ground
- Active fault code SPN 3243 FMI 4 logged in ACM
- DEF dosing system inhibited or restricted
- DPF regeneration inhibited due to DEF system fault
- Dashboard warning lamp or SCR/DEF indicator illuminated
- Possible derate or engine power reduction depending on OEM threshold
Root Causes
- Sensor signal wire shorted to ground in wiring harness
- Failed or internally shorted DEF tank temperature sensor 2
- Corroded or damaged connector at DEF tank temperature sensor 2
- Chafed wiring harness causing short to chassis ground
- Water intrusion into connector causing low resistance fault
- ACM sensor input circuit failure (rare)
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 3243 FMI 4 will typically inhibit or restrict DEF dosing due to loss of accurate DEF tank temperature data, which in turn can inhibit or delay active DPF regeneration. Prolonged fault may trigger SCR system derate depending on OEM escalation thresholds.
System Interactions
SPN 3243 FMI 4 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 3243 FMI 4 — 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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