when in auto boiler permenantly on ?

have changed my transmitter and receiver 3 times now through my heating engineer, the problem is the transmitter when in auto mode stays permanently on and ignores the time settings no matter what... it seems to work ok for a few days then just does the same thing again...lights on receiver working fine heating good and plenty of hot water...have done a reset a number off times ..but three units I have replaced cannot all be at fault..could there be a interference from a outside source ...driving me crazy !!!!

Dave
January 2015