Kenmore 16.7 cu upright frost free freezer

Question

I have the freezer on the lowest setting but things are rock hard and a thin layer of frozen water in on the bottom of the freezer. I thawed the freezer this past summer so the drain hole was open again. Yesterday I noticed that it is froze up again. I have read your advice on other postings with problems like mine but you only say what is going on, which I know the drain hole is froze up probably because the freezer is too cold. One posting stated that the temp control board or thermistor is not reading the temp. How hard is that to replace? My husband is pretty good with repairs but we don't know where to start or if we should just call a repair man. Thank you for your help with this problem. Of course the 5 year warranty has just run out for the sealed system(refrigerant, evaporator and condenser) but not sure it would fall under those repairs.