From the interwebs:
"Code U0155 is typically caused by one of the following:
- A dead battery
- A faulty instrument cluster
- A problem with the instrument cluster circuit
- A problem with the CAN bus"
I'd try replacing the battery and go from there. Seems to be the cheap easy attempt to fix.
EDIT: Scratch that, a quick search:
https://www.ft86club.com/forums/showthread.php?t=18511
EDIT 2: Or maybe a new battery LOL:
https://www.ft86club.com/forums/showthread.php?t=141112