Safe to driveEnough to reach the yard
J1939 code SPN 39. Below — how the fault is worded at the factory by Ford Trucks and ZF AS-Tronic, 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 0 | Outside cab engine start start button stuck. FODP code (scan tool): U0555-00. |
| FMI 1 | Outside cab engine start stop button stuck. FODP code (scan tool): U0555-01. |
ZF AS-Tronic uses its own fault numbering: the number in the table below is an internal ZF code, not a J1939 SPN. It matches the title of this page by digit only and means something different.
| FMI 10 | Fault on valve Y7 output circuit (engagement valve). |
| SPN 39 FMI 039/0 · 39.0 | data above normal range (most severe level) |
| SPN 39 FMI 139/1 · 39.1 | data below normal range (most severe level) |
| SPN 39 FMI 1039/10 · 39.10 | abnormal rate of change |
Safe to drive. Below — how much time you have and what happens if you keep going.
The modules have stopped hearing each other on the bus. Some dash readings are now made up and some functions are simply off.
You can physically drive, but do not trust the instruments, and the other codes in the list may well be false: fix the bus first, then look at the rest.
That said, the manufacturer flags this code as critical — if in doubt, it is safer to stop and check on the spot.
Check connectors and the harness: a broken CAN twisted pair takes out communication for every ECU at once.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.