Samsung SR-S2028CVW “flashing -20 all time and defrosting”

Hi,Samsung fridge/freezer SR-S2028CVW runs ok first few hours when plugged in achieves temperature in both fridge and freezer(set at -20 and 3)but will then completely stop refrigerating in both fridge and freezer,compressor knocks off incl both evap fan motors and fascia displays -20 degrees but flashes(even quite clearly not that temp)and fridge showing 3 (but not flashing).Any pointers towards problem would be grateful.Also pressed top two buttons in which i have been told does a diagnostic and reveals nothing,just flashes all increments for a few seconds then goes blank.Wondering if something ocurrent/non ocurrent whilst defrosting and not achieving defrost termination temp thus knocking system off??Any info would be appreciated,thanks.