SPN 1323 FMI 31Engine
Engine Misfire, Severe - Condition Exists
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 1323 / FMI 31
Category
Engine
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Misfire detection is monitored by the ECM using crankshaft position sensor data and individual cylinder contribution analysis.
Common Symptoms
- Severe engine vibration or shaking
- Loss of power and acceleration
- Rough idle with noticeable cylinder miss
- Check engine light illuminated
- Engine derate or limp mode activated
- Excessive white or black smoke from exhaust
Root Causes
- Faulty fuel injector or injectors Low fuel pressure or fuel supply issue Air in fuel system Faulty engine wiring harness or connector ECM detecting multiple cylinder contribution imbalance
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Derate Behavior
Power Derate
Yes
System Interactions
SPN 1323 FMI 31 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 1323 FMI 31 — 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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