What are the symptoms of SPN 639 FMI 6?

Network issues, erratic module behavior

What causes SPN 639 FMI 6?

Short circuit in J1939 CAN bus wiring harness Damaged or pinched CAN high or CAN low wires Faulty ECM or module causing excessive current draw on network Corroded or moisture-damaged CAN bus connectors Improperly terminated CAN network or incorrect terminating resistors

How do I diagnose SPN 639 FMI 6?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 639 FMI 6 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 639 FMI 6Network / CANHigh

J1939 Network - Current Above Normal (CAN Short Circuit)

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Overview

Fault Code

SPN 639 / FMI 6

Common Symptoms

Network issues, erratic module behavior

Root Causes

Short circuit in J1939 CAN bus wiring harness Damaged or pinched CAN high or CAN low wires Faulty ECM or module causing excessive current draw on network Corroded or moisture-damaged CAN bus connectors Improperly terminated CAN network or incorrect terminating resistors

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

CAN short circuit — disconnect modules one at a time to isolate.

System Interactions

SPN 639 FMI 6 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

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3251-0DPF Differential Pressure High SPN 157-18Fuel Rail Pressure Low

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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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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