SPN 304 FMI 0Ignition/MisfireHigh
Cylinder 4 Misfire Detected
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 304 / FMI 0
Category
Ignition/Misfire
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 4 fuel injector is located on the engine block fourth cylinder position, with associated wiring harness routed from the ECM.
Common Symptoms
- Rough idle or engine vibration at low RPM
- Loss of power or reduced acceleration
- Increased fuel consumption
- Black or unburned fuel smoke from exhaust
- Check engine light or amber warning lamp illuminated
- Engine may enter derate mode under load
Root Causes
- Faulty or worn fuel injector on cylinder 4 Low compression in cylinder 4 due to worn rings or valve seating issue Failed or degraded glow plug on cylinder 4 (if applicable) Restricted or leaking fuel supply to cylinder 4 Damaged or cracked cylinder 4 injector return line or o-ring
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Derate Behavior
Mileage / Regen Notes
Active and parked DPF regeneration will be inhibited or aborted when a cylinder misfire is detected, as unburned fuel entering the DPF can cause overtemperature damage or DPF failure.
System Interactions
SPN 304 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 304 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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