SPN 651 FMI 6Fuel SystemHigh
Injector Cylinder 1 - Current Above Normal
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Overview
Fault Code
SPN 651 / FMI 6
Category
Fuel System
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 1 injector is mounted in the cylinder head at the number 1 cylinder position, typically at the front of the engine on inline configurations.
Common Symptoms
- Rough running or misfiring, especially noticeable at idle. Engine may run on fewer cylinders (loping idle). Excessive black smoke from exhaust. Possible knocking or abnormal combustion noise from cylinder 1. Check engine light illuminated. Reduced power and poor throttle response. In severe cases, engine may shut down to protect ECM driver circuits. Elevated exhaust temperatures on affected cylinder bank.
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 associated misfire and rough running may cause the ECM to suspend regen due to unstable engine operation or abnormal exhaust temperatures. If the fault causes significant power derate, the engine may not sustain the load or RPM required for active regen. Resolve the injector circuit issue first, then verify regen capability with a forced regen test if soot load is elevated.
System Interactions
SPN 651 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 TSBs — verify in OEM service portal. Common areas to search: injector harness chafing bulletins, valve cover grommet seal updates, and injector connector corrosion advisories. Cummins ISX and ISB platforms have known harness routing concerns. Detroit DD13/DD15 have issued injector harness service kits for valve cover area.
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 651 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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