SPN 305 FMI 0Ignition/MisfireHigh
Cylinder 5 Misfire Detected
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Overview
Fault Code
SPN 305 / FMI 0
Category
Ignition/Misfire
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 5 is located on the engine block, with injector, valve train, and combustion components accessible from the valve cover.
Common Symptoms
- Rough idle or engine misfire felt at low RPM
- Loss of power or hesitation under load
- Increased fuel consumption
- MIL or Check Engine lamp illuminated
- Visible black or unburned exhaust smoke
- Engine vibration noticeable in cab
Root Causes
- Faulty or worn fuel injector on cylinder 5 causing improper fuel delivery
- Low compression in cylinder 5 due to worn rings, valves, or head gasket failure
- Failed or degraded glow plug on cylinder 5 affecting cold-start combustion
- Air in fuel system causing inconsistent combustion events
- Wiring harness fault or connector corrosion at cylinder 5 injector circuit
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Derate Behavior
Mileage / Regen Notes
Active cylinder 5 misfire will inhibit active DPF regeneration to prevent raw fuel from damaging the DPF substrate; regen lockout will persist until misfire fault is resolved.
System Interactions
SPN 305 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 305 FMI 0 — 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.
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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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