FMI 25 — Reserved for SAE assignment
SAE J1939 reserves this failure mode for future assignment, so a healthy truck should never report it. If your reader shows it, the problem is almost always the translation between the bus and your tool — a proprietary code being force-fit into the J1939 FMI field, a bad adapter, or a corrupted message.
Common causes
- Scan tool or app mistranslating a manufacturer-proprietary fault into a J1939 FMI
- Cheap or failing Bluetooth/USB adapter corrupting the message
- Bus noise or a wiring fault producing a garbled frame
- Non-compliant aftermarket module broadcasting on J1939
What to test, in order
- Re-read the codes with a second tool or an OEM tool and see whether the same FMI comes back
- Check J1939 bus health: about 60 ohms across the datalink pins with the key off, clean backbone and stub connectors
- Unplug aftermarket telematics, ELDs and dongles, then re-scan
- If a different tool shows a normal FMI, work that code and ignore the reserved reading
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