What is SPN 639 FMI 9 & How to Solve It?

What is SPN 639 FMI 9:

SPN 639 refers to the Controller Area Network (CAN) communication link, and FMI 9 means the update rate is abnormal — meaning the engine ECM is not receiving consistent data from other modules.

This code indicates a communication delay or dropout between control units, which may affect the engine, transmission, or aftertreatment operations.

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Why This Happens

• Intermittent wiring connection on the CAN network.
• Faulty or loose control module connector.
• Damaged wiring harness between ECUs.
• Electrical noise or low system voltage.
• Control module internal fault.

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How to Fix It

1. Inspect CAN network wiring for signs of wear or pinching.
2. Check connectors at major ECUs for looseness or corrosion.
3. Perform voltage and resistance checks across the CAN lines.
4. Secure or replace damaged sections of the harness.
5. If wiring checks out, run diagnostics to isolate which module is not responding properly.
6. Clear the code and test system communication again.

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How to Avoid It Before It Happens

1. Keep control module connectors clean and tightly secured.
2. Schedule periodic voltage and continuity checks on CAN wiring.
3. Document any intermittent issues or network-related warnings.
4. Protect wiring looms from vibration or heat stress.