What is SPN 639 FMI 7 & How to Solve It?

What is SPN 639 FMI 7:

SPN 639 is a CAN communication fault, and FMI 7 means the system detected a mechanical issue or degraded performance in message handling.

This often reflects poor data transmission, delayed commands, or message loss between modules.

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Why This Happens

• CAN wiring is degraded, bent, or stretched.
  
• Terminating resistor value is incorrect.
  
• Signal interference from other electronics.
  
• Poor voltage supply to a connected module.
  

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How to Fix It

1. Test total network resistance (should equal 60 ohms).
   
2. Inspect CAN wiring for kinks, heat damage, or wear.
   
3. Replace or repair any damaged connector.
   
4. Update firmware on control modules.
   
5. Clear the code and verify message timing and response.
   

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How to Avoid It Before It Happens

1. Use diagnostic tools to monitor CAN bus latency.
   
2. Replace aged or brittle network harness sections.
   
3. Keep modules well-grounded and voltage-stable.
   
4. Test CAN resistance during every major inspection.