What is SPN 639 FMI 2 & How to Solve It?

What is SPN 639 FMI 2:

SPN 639 refers to the Controller Area Network (CAN) Communication Link, and FMI 2 indicates that the data is erratic, intermittent, or incorrect.

This code appears when a control module (such as the ECM, transmission controller, or aftertreatment module) is sending inconsistent or unstable data over the CAN network, which can disrupt overall machine operation or diagnostics.

‍

Why This Happens

• Loose or corroded CAN connectors.
• Damaged wiring causing signal dropouts.
• Failing or rebooting control module.
• Grounding issues leading to unstable signals.
• Electrical interference or poor voltage regulation.

‍

How to Fix It

1. Use diagnostics to identify which module is sending erratic data.
2. Inspect all CAN connectors for corrosion, looseness, or damage.
3. Test wiring continuity and resistance across the CAN network (should total 60 ohms).
4. Replace or reflash any module that is resetting or failing intermittently.
5. Clear the fault and retest system communication stability.

‍

How to Avoid It Before It Happens

1. Keep connectors dry, clean, and properly sealed.
2. Include CAN resistance testing in your PM routine.
3. Replace harness sections showing wear or heat damage.
4. Document and address any repeated module resets.

‍