SPN 656 FMI 5Fuel SystemHigh
Injector Cylinder 6 — Current Below Normal or Open Circuit
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 656 / FMI 5
Category
Fuel System
Fault Type
electrical
FMI Category
current low
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Cylinder 6 injector — rearmost cylinder on inline-6 engines. Injector harness runs full length of valve cover rail to rear of engine. Connector at top of valve cover near injector body adjacent to flywheel housing.
Common Symptoms
- Cylinder 6 misfire — rough idle and uneven power delivery
- Engine knock or rattle from cylinder 6 (rear) position
- Reduced power and fuel economy
- Black or white smoke from exhaust
- Hard start or extended crank time
- MIL illuminated with active injector fault
Root Causes
- Injector solenoid open circuit — internal injector winding failure
- Broken or chafed injector harness wire at cylinder 6
- Corroded or backed-out injector connector at cylinder 6
- ECM injector driver circuit failure for cylinder 6 channel
- Injector return circuit open — missing ground path
- Injector body failure — mechanical seizure causing electrical open
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
Power Derate
Yes
Mileage / Regen Notes
Injector solenoid winding failure increases above 400,000 miles. Rear cylinder harness chafing risk is highest due to harness length and proximity to flywheel housing.
System Interactions
SPN 656 FMI 5 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: Cummins TSB 4021756 — Injector Solenoid Circuit Diagnosis; Detroit TSB 16-008 — Injector Open Circuit Diagnosis
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.
OEM Notes
Cummins
ISX/X15: If SPN 654, 655, and 656 are all active, suspect ECM driver bank B failure. INSITE injector test required. Cylinder 6 harness is longest — check full length for damage.
Detroit
DD15: Cylinder 6 injector connector corrosion is common. Clean connector before replacing injector. DDDL required.
PACCAR
MX-13: Confirm with DAVIE4 injector test. Cylinder 6 harness routing near flywheel housing is a known chafe point.
Volvo
MP8: PTT injector contribution test required. Rear cylinder harness routing near flywheel housing is a chafe risk.
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 5 — 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 656 FMI 5
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.