SPN 631 FMI 12Fuel System
Engine Injector Cylinder 5 - Bad Intelligent Device or Component
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Overview
Fault Code
SPN 631 / FMI 12
Category
Fuel System
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 5 fuel injector is mounted in the cylinder head on the fifth cylinder from the front of the engine.
Common Symptoms
- Engine misfire on cylinder 5
- Rough idle or uneven engine operation
- Loss of power or reduced engine performance
- Increased fuel consumption
- White or gray smoke from exhaust
- Check engine light illuminated
Root Causes
- Failed injector internal electronics or solenoid driver Corrupted injector calibration data or trim codes Faulty injector communication circuit Damaged injector connector or terminal corrosion ECM software compatibility issue with injector version
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Derate Behavior
Power Derate
Yes
System Interactions
SPN 631 FMI 12 interacts with the following systems and may trigger cascading faults if not resolved promptly.
Fuel rail pressure — injector timing and quantity affected
Engine power output — derate may be triggered to protect injectors
Aftertreatment regen — low fuel pressure can prevent regen completion
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.
Replacing injectors without verifying fuel supply pressure — low supply pressure causes injector faults.
Ignoring fuel filter restriction — a clogged filter causes low rail pressure that mimics injector failure.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 631 FMI 12 — 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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