SPN 51 FMI 2 – J1939 Network Error [Troubleshooting Guide]

What Does SPN 51 FMI 2 Mean?

  • SPN (Suspect Parameter Number): 51

  • FMI (Failure Mode Identifier): 2 = Data Erratic, Intermittent, or Incorrect

  • Translation: The ECM has detected corrupted or unstable communication on the J1939 data link.

This often means the truck’s control modules are not properly communicating due to wiring issues, voltage spikes, or faulty components.

Common Causes of SPN 51 FMI 2

  • Damaged or corroded J1939 CAN wiring

  • Loose or dirty connections at the 9-pin diagnostic port

  • Faulty ECM, TCM, or ABS module

  • Bad terminating resistor

  • Interference from aftermarket electronics

  • Improper voltage on the CAN high / low circuits

How to Troubleshoot SPN 51 FMI 2

  1. Inspect 9-Pin Diagnostic Port

    • Look for corrosion, bent pins, or loose fit.

  2. Check J1939 Wires

    • Measure resistance between CAN Hi and CAN Lo at 60 ohms.

    • Inspect for damaged shielding or shorts to ground.

  3. Scan with a Diesel Laptop

    • Use a diagnostic scanner (like our Diesel Tech or Universal Diag).

    • Run a network health report and log any additional SPN/FMI codes.

  4. Check Terminating Resistors

    • There should be two 120-ohm resistors (typically at the ends of the CAN bus).

    • Faulty or missing resistors will throw communication errors.

  5. Look for Module-Specific Faults

    • If the ECM can’t “see” another module (e.g., ABS), that component may be offline or dead.

Recommended Tools

Real-World Fix Example:

A 2016 Freightliner Cascadia showed SPN 51 FMI 2 and random dashboard glitches. Using a diesel laptop, the tech traced the issue to a chafed J1939 wire behind the driver kick panel. Repairing the wire and replacing one faulty terminating resistor cleared the code.

Pro Tip:

If this fault is intermittent, check for loose aftermarket devices tapping into the CAN bus (like ELDs or GPS trackers). Remove them temporarily to isolate the issue.

Conclusion

SPN 51 FMI 2 points to a J1939 network communication issue. Start with the basics—check the diagnostic port and CAN wiring, then use your diesel laptop to run a deeper scan. With the right tools, this fault is fully diagnosable and fixable in-house.

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