SPN 300 FMI 0Ignition/MisfireHigh
Random/Multiple Cylinder Misfire Detected
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Overview
Fault Code
SPN 300 / FMI 0
Category
Ignition/Misfire
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: Misfire detection is derived from crankshaft speed variation; crank sensor is typically mounted on lower engine block near the flywheel housing.
Common Symptoms
- Rough or uneven engine idle
- Loss of power or reduced acceleration
- Excessive black or white exhaust smoke
- Engine vibration felt in cab
- MIL or Check Engine lamp illuminated
- Poor fuel economy reported by driver
Root Causes
- Faulty or worn fuel injector(s) causing uneven cylinder contribution
- Low cylinder compression due to worn rings, valves, or head gasket failure
- Air in fuel system or restricted fuel supply causing lean misfire
- EGR system malfunction causing excessive dilution of combustion charge
- Camshaft or valve train wear affecting injection timing on affected cylinders
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Derate Behavior
Mileage / Regen Notes
Active and passive DPF regeneration will be inhibited when misfire is detected due to risk of unburned fuel loading the aftertreatment system; soot load may increase rapidly and force a parked regen requirement.
System Interactions
SPN 300 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 300 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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