After inserting the batteries and switching on the thermostat, the display illumination stays on for 4-5 seconds, after which the screen turns black and nothing can be set, from programming to screen control, you always have to press a button (the M button) to switch the display illumination back on, and after 4-5 seconds it switches off again (the thermostat is still on). Are there any special settings to be made to keep the light on for longer, or is it a defect and should I return the product? Thanks for the answers! Sebasiano