SPN 639 FMI 5Network / CANHigh
J1939 Network - Current Below Normal (CAN Open Circuit)
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Overview
Fault Code
SPN 639 / FMI 5
Category
Network / CAN
Severity
High
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: CAN bus wiring runs throughout the vehicle chassis connecting the ECM to other J1939 network modules and components.
Common Symptoms
- Communication issues, loss of module data
Root Causes
- Open circuit in CAN bus wiring harness Damaged or corroded CAN bus connector Missing or faulty terminating resistor on CAN network Broken wire at ECM CAN connection point Failed CAN transceiver in ECM or module
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
CAN open circuit — check resistance and terminators first.
System Interactions
SPN 639 FMI 5 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.
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 639 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.
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