SPN 308 FMI 0Ignition/MisfireHigh
Cylinder 8 Misfire Detected
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Connector Pinout — in guided session
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Overview
Fault Code
SPN 308 / FMI 0
Category
Ignition/Misfire
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 8 is the rearmost cylinder on the driver-side bank of a V-configuration diesel engine, accessible from the valve cover.
Common Symptoms
- Rough idle or engine vibration at idle
- Loss of power or reduced engine performance
- Check engine light illuminated
- Increased fuel consumption
- Engine stumble or hesitation under load
- Visible black or white smoke from exhaust
Root Causes
- Failed or worn fuel injector on cylinder 8 causing improper fuel delivery
- Low cylinder compression due to worn rings, damaged valves, or blown head gasket
- Faulty or stuck exhaust brake/engine brake on cylinder 8
- Wiring harness damage or connector corrosion at cylinder 8 injector
- Camshaft or rocker arm wear affecting cylinder 8 valve timing
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Derate Behavior
Mileage / Regen Notes
Active cylinder 8 misfire will inhibit or abort DPF active regeneration due to unburned fuel risk in the aftertreatment system; regen lockout will persist until misfire fault is resolved.
System Interactions
SPN 308 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 308 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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