SPN 3243 FMI 18Aftertreatment
Aftertreatment Fuel Rate - Data Valid But Below Normal
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Overview
Fault Code
SPN 3243 / FMI 18
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Aftertreatment fuel system components (doser injector, metering valve, fuel pump) are located on the exhaust aftertreatment system, typically on or near the DPF/DOC assembly under the vehicle chassis.
Common Symptoms
- Insufficient DPF regeneration due to low fuel dosing
- DPF regeneration fails to complete or takes excessive time
- Aftertreatment outlet temperature lower than expected during regen
- Check engine light illuminated
- Active regeneration requests more frequently than normal
- High DPF soot load accumulation
Root Causes
- Restricted or clogged aftertreatment fuel injector (doser injector) reducing fuel delivery rate Low fuel supply pressure to the aftertreatment fuel system Faulty or sticking aftertreatment fuel metering valve Air ingestion or aeration in the aftertreatment fuel supply line Partially blocked aftertreatment fuel filter or strainer Faulty aftertreatment fuel pump delivering insufficient pressure/flow Leaking or cracked aftertreatment fuel supply line causing loss of prime ACM software or calibration issue causing incorrect fuel rate calculation
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
An active SPN 3243 FMI 18 will inhibit or abort active DPF regeneration because insufficient aftertreatment fuel rate prevents proper DOC/DPF heat generation. Stationary and in-service regens may be blocked until the fault is resolved.
System Interactions
SPN 3243 FMI 18 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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3243 FMI 18 — 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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