SPN 640 FMI 9Electrical/J1939High
J1939 Network 2 — Abnormal Update Rate
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 640 / FMI 9
Category
Electrical/J1939
Fault Type
sensor
FMI Category
communication
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: J1939 secondary CAN bus — throughout vehicle wiring harness
Common Symptoms
- J1939 secondary CAN bus fault
- Module communication fault
- Engine derate
Root Causes
- Secondary CAN bus termination fault
- CAN bus wiring damage
- Module communication fault
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
CAN bus faults can occur at any mileage.
System Interactions
SPN 640 FMI 9 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021241 — J1939 Secondary CAN Bus 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.
OEM Notes
Cummins
X15: Secondary CAN bus connects body controller and other modules.
Detroit
DD15: DDDL shows secondary CAN bus status.
PACCAR
MX-13: Check CAN bus termination at both ends.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 640 FMI 9 — 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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