UD Sync is an encoder issue. ( UD 5y nc )
The board is not installed correctly on the unit causing the encoder not to be alignedwith the slotted gear.
Slotted gear is not centered between the two sensors on the backside of the board, Issuewith slotted gear, a slot plugged with debris or a broken slot.
Capacitor wasn’t drain erasing last know position the valve currently is not in. Unplug unit and hold black reset button on the back of the board down for 10seconds. Plug back in.
UD Sync is the 2 sensor eyes cannot read the slotted gear that passes between them.
UD Sync means it is looking for service position.
If it cannot find it the timer will stay in UD sync.