SPN 303 FMI 0Ignition/MisfireHigh
Cylinder 3 Misfire Detected
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.
Overview
Fault Code
SPN 303 / FMI 0
Category
Ignition/Misfire
Severity
High
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 3 injector is located in the third cylinder position on the engine block, with associated wiring routed through the engine harness.
Common Symptoms
- Rough idle or engine roughness especially at low RPM
- Loss of power or reduced engine performance
- Excessive exhaust smoke (black or white) from cylinder 3 misfire
- Engine vibration felt through cab or chassis
- Increased fuel consumption
- MIL or Check Engine lamp illuminated with active fault
Root Causes
- Faulty or failed fuel injector on cylinder 3 (stuck open, clogged, or erratic delivery)
- Low compression on cylinder 3 (worn rings, damaged valves, or head gasket failure)
- Failed or degraded glow plug on cylinder 3 causing cold-start misfire
- Air in fuel system or restricted fuel supply affecting cylinder 3 delivery
- Wiring harness damage or connector fault at cylinder 3 injector circuit
Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.
AI walks you through every test — one step at a time. Plans start at $12.95/mo.
Derate Behavior
Mileage / Regen Notes
Active cylinder 3 misfire will typically inhibit or abort DPF active regeneration due to unburned fuel risk and elevated exhaust HC levels; regen lockout remains until fault is resolved.
System Interactions
SPN 303 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 303 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.
Get weekly diesel diagnostic tips
Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.
No spam. Unsubscribe anytime.
Get AI-Guided Diagnosis for SPN 303 FMI 0
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.