The fire detector may be faulty or simply activated and in need of a reset.
If you cant clear it by entering your alarm code and resetting in the normal way you need to call an engineer.
If you want to disable it completely
Remove mains power
Remove back up battery inside control panel
If at this point the external siren activates you need to disable the battery in the external alarm box.
Be careful as the control panel has mains electricity inside and be careful on the ladder too !
Alarm Engineer Bristol
July 2008