SPN 3243 FMI 2Aftertreatment
DPF Differential Pressure - Data Erratic or Intermittent
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Overview
Fault Code
SPN 3243 / FMI 2
Category
Aftertreatment
Derate Risk
No
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 along the exhaust system under or behind the cab.
Common Symptoms
- Check engine light or MIL illuminated
- Engine derate or reduced power
- Excessive forced regeneration requests
- Erratic DPF pressure readings on scan tool
- Aftertreatment system warnings on dash
- Intermittent fault code appearance
Root Causes
- Plugged, kinked, or collapsed differential pressure sensor hoses/tubes DPF differential pressure sensor contaminated with soot or moisture Loose or corroded connector at the differential pressure sensor DPF differential pressure sensor internal failure Air leak at pressure tap ports on DPF inlet or outlet Damaged or cracked pressure sensing lines High soot loading causing erratic pressure readings ACM software or calibration issue causing erratic signal interpretation
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
FMI 2 (data erratic/intermittent) on the DPF differential pressure sensor will typically inhibit active DPF regeneration because the ACM cannot reliably determine DPF soot load. The system may also disable parked regen requests and illuminate the DPF warning lamp. Passive regen may continue under certain conditions depending on OEM strategy.
System Interactions
SPN 3243 FMI 2 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 2 — 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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