it could be the battery in the control unit,it prob has not been changed since it was first fitted,they dont last forever,before you open the end station,remove the fuse in the spur unit that feeds mains power,or if no spur,turn mains power off before you open the end station ,you cant test the battery with a test meter,we use a device that puts a dummy load on the battery,so just buy the same type of battery you find inside the unit
gts, December 2012