GARDTEC 300/500 ALARM?

Gardtec 300/500 = 360/580 Alarm
Showing Ct on the panel. If I take the fuse out and set the alarm going then reconnect the fuse, the alarm stops and shows Un. When I come to set the alarm to go out and put in the code the alarm goes audible as it is supposed to but does NOT set, and stays audible ?
Once you again try to reset this you go back to Ct.
:-(
Can anyone please help.