SPN 3248 FMI 4AftertreatmentMedium
Aftertreatment 1 SCR Inlet Temperature — Voltage Below Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3248 / FMI 4
Category
Aftertreatment
Fault Type
sensor
FMI Category
voltage low
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: SCR inlet temperature sensor — on exhaust pipe upstream of SCR
Common Symptoms
- SCR inlet temp sensor reading low
- DEF dosing affected
- SCR efficiency monitoring unavailable
Root Causes
- Sensor open circuit
- Wiring open circuit
- Sensor failed
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Open circuit faults common at 200k–350k miles.
System Interactions
SPN 3248 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
TSB Reference: Cummins TSB 4021941 — SCR Inlet Temperature Sensor Open Circuit
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: Check harness routing near SCR.
Detroit
DD15: Check connector for moisture.
PACCAR
MX-13: Inspect connector boot.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3248 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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