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Fault Code Reference

SPN 627 FMI 4Fuel SystemMedium

Injector Power Supply - Voltage Low

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Overview

Fault Code

SPN 627 / FMI 4

Category

Fuel System

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: Injector power supply circuit originates at the ECM, routed through the engine harness to fuel injector connectors located on the valve cover or cylinder head.

Common Symptoms

  • Multiple injector fault codes present (often all cylinders), rough running or misfiring on multiple cylinders, hard starting or extended crank time, black smoke at idle and under load, engine may run on fewer cylinders or not start at all, reduced power complaint, engine shaking or severe vibration at idle, possible no-start condition if voltage drops too low

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

Active DPF regeneration is typically inhibited while this fault is active. The ECM will not command a regen with unstable injector power supply because proper fuel delivery is required for in-cylinder dosing or 7th injector operation. Expect regen to be locked out until fault is resolved and code is cleared. Parked regen will also likely be blocked. Resolve injector supply issue before attempting any forced regen.

System Interactions

SPN 627 FMI 4 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 ISX/ISB: Check for TSBs related to injector harness chafing near valve cover. Detroit DD13/DD15: Check for bulletins on injector harness connector corrosion. Paccar MX: Check for injector loom routing updates. Always search OEM QuickServe, DTNA, or Paccar portals for latest bulletins by specific engine serial number.

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.

Deep-dive diagnostic page: SPN 627 FMI 4 — 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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