Safe to driveTo the nearest safe stop
J1939 code SPN 684. Below — how the fault is worded at the factory by Ford Trucks, and what the second half of the code (FMI) means in the standard.
Ford Trucks/Ecotorq writes fault numbers in its own format: the code in the table below is a Ford code (hexadecimal, zero-padded), not a J1939 SPN. It matches the standard only where the signal itself is standard, and usually matches the title of this page by digit alone.
| FMI 2 | Control Module Communication Bus “D” Off |
| SPN 684 FMI 2684/2 · 684.2 | data erratic or intermittent |
Safe to drive. Below — how much time you have and what happens if you keep going.
The gearbox or clutch has gone into limp mode: some gears may be unavailable and an automated box may stop shifting under load.
The main risk is being left on a climb with no gear and rolling back. Do not take a loaded truck into the hills; reaching level ground and a shop is fine.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.