Is it the display screen that is faulty? How do you know?
Double check the batteries are fresh, that they are correctly inserted and that the positive pips aren't oversized and preventing the positive contacts being completed (I mention that as I've been had like that once and chased the problem around for ages).
I seem to remember a Drayton or Danfoss thermostat had a recall once because the battery tray had problems - could be an issue with yours but I am blowed if I can remember any more details than that.
acp, April 2017
