Hi, I moved house last year and the alarm which was previously installed in the flat was working well, albeit no manual but i used an online help to check basic function. From what i can tell from the online manual (as it doesn't have a model on the alarm itself) it is a Challenger AP200. All of a sudden I am no longer able to set the alarm by typing in the 4 digit code. when I do this, one of the zone lights remains on and when i exit the tone remains the same and the alarm won't set. To get rid of the noise or 're-set' it I merely have to press any key and the tone will cease. I think the problem is with zone 1 as this is the light which remains on, which is one of my eternal doors. I've had a look to see where the sensor/s are and think they are on the top of the door but no idea how to fix - any ideas? I live on my own and set the alarm at night so really need to get it fixed asap, thanks for your help