SPN 656 FMI 6Fuel SystemHigh
Injector Cylinder 6 - Current Above Normal
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Overview
Fault Code
SPN 656 / FMI 6
Category
Fuel System
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 6 injector is mounted in the cylinder head at the number 6 cylinder position, typically the rearmost cylinder on inline-6 engines or rear of bank on V-configurations.
Common Symptoms
- Misfire on cylinder 6, rough idle, excessive black smoke, loss of power, engine running rough especially under load, possible white smoke at startup, elevated exhaust temperature on cylinder 6 bank, audible knocking or uneven engine sound, fuel in crankcase oil if injector stuck open
Root Causes
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
This fault typically does not directly inhibit active DPF regeneration. However, the resulting cylinder 6 misfire can cause incomplete combustion and unburned fuel entering the exhaust, which may trigger additional aftertreatment faults or cause elevated DOC/DPF inlet temperatures. If the ECM detects unstable engine operation due to the misfire, it may postpone or abort active regen as a protective measure. Parked regen may still be allowed but is not recommended until the injector circuit issue is resolved to prevent aftertreatment damage.
System Interactions
SPN 656 FMI 6 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
TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins, Detroit, and Paccar have issued injector harness chafing bulletins for specific model years — search by engine serial number. Check for any harness routing or connector upgrade kits applicable to your engine platform.
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
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 656 FMI 6 — 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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