SPN 629 FMI 2Communication
CAN Data Bus 1 - Data Erratic, Intermittent, or Incorrect
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Connector Pinout — in guided session
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Overview
Fault Code
SPN 629 / FMI 2
Category
Communication
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: CAN bus wiring runs throughout the vehicle chassis connecting all electronic control modules via twisted pair data lines.
Common Symptoms
- Intermittent loss of gauge cluster data or flickering displays
- Erratic speedometer or tachometer readings
- Multiple unrelated fault codes active simultaneously
- Intermittent loss of communication with ECU modules
- Engine protection modes activating sporadically
Root Causes
- Damaged or corroded CAN bus wiring harness connections Poor quality or loose CAN bus termination resistors Faulty ECU module causing data corruption on the network Water intrusion in CAN bus connectors Electromagnetic interference from aftermarket accessories
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Derate Behavior
Power Derate
Yes
System Interactions
SPN 629 FMI 2 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
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 629 FMI 2 — 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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