SPN 629 FMI 3Turbo
VGT Actuator Position - Voltage Above Normal
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 629 / FMI 3
Category
Turbo
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: The VGT actuator is mounted on the turbocharger housing, typically on the exhaust side of the turbocharger assembly.
Common Symptoms
- Check engine light illuminated
- Loss of engine power or derate
- Reduced turbo boost pressure
- Black smoke under acceleration
- Poor throttle response
Root Causes
- Faulty VGT actuator or position sensor Damaged wiring harness to VGT actuator Corroded or loose connector at VGT actuator Failed ECM internal driver circuit Shorted circuit in actuator feedback wiring
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Derate Behavior
Power Derate
Yes
System Interactions
SPN 629 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 629 FMI 3 — 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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