If Zephyr DCS50/DCS51’s display shows FuLL this means that it has reached the limit of 10 (20 for the Zephyr Xtra) locomotive addresses that it can manage at one time. 
If you want to select additional addresses, you will need to release one or more loco addresses before you will be able to select other locomotives to run.
The DCS50/DCS51 runs a “purging” routine that helps eliminate addresses that have not been used for a period of time to make slots available for running other addresses.
If you see this "FuLL" message while you are operating, you can release locos that you are no longer using by pressing the LOCO key, entering the address you want to release and then pressing the exit key.
If you want to clear all the addresses and consist information from your DCS50/DCS51 and start over you can set Option Switch 36 to closed as follows:
1. Be sure everything on the layout is at 0 speed.
2. Press the PROG key.
3. Press the SWITCH key.
4. Enter 036. ** NOTE ** If using a DCS51 Zephyr Xtra, Enter 039 instead of 036.
5. Press the c/- key.
6. Press the EXIT key.
Your DCS50/DCS51 will re-initialize with no locomotives selected and no consists set up to run. Any locomotives that may have been operating at the time of the reset will come to a stop.