First of all when changing batteries after that much time you need to change them all , and don't buy cheap rubbish either , Duracell Energizer's etc
You will need an engineer most probably , if you don't have engineers access
But the user log should tell you what's wrong , why don't you Google the user manual and have a look, without no detail it could be anything
Make your decision on how much you value what your protecting , if it's usual , you have nothing or not much and not bothered , just thought you give a go
Rip off wall and as Mr H might say buy a dog?