SPN 3243 FMI 3Aftertreatment
DPF Differential Pressure - Voltage Above Normal
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Overview
Fault Code
SPN 3243 / FMI 3
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: DPF differential pressure sensor is mounted on or near the DPF canister, typically on the aftertreatment assembly mid-to-rear chassis, with hose ports connecting upstream and downstream of the DPF.
Common Symptoms
- Check engine light illuminated
- Engine derate or power reduction
- DPF regeneration frequency increased
- Exhaust system fault warning displayed
- Poor fuel economy
Root Causes
- Shorted or damaged wiring harness between DPF differential pressure sensor and ACM (wire shorted to voltage source) Failed DPF differential pressure sensor with internal short causing high voltage output Corroded or moisture-contaminated sensor connector causing voltage spike Damaged ACM sensor input circuit (rare, rule out external causes first) Incorrect sensor installed or sensor reference voltage line shorted to signal line
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
SPN 3243 FMI 3 (DPF differential pressure voltage above normal) will typically inhibit active DPF regeneration and may trigger a regen inhibit lamp. The ACM cannot accurately determine soot load without valid differential pressure data, preventing safe regen initiation.
System Interactions
SPN 3243 FMI 3 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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3243 FMI 3 — 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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