Samsung RSH5UBBP defrosts with no alarm

Hi just had the 4th engineers visit to fix the above fault and now had a call from Samsung saying that there is nothing wrong with it as no error codes appear when the engineer came out and i would get a call from further up the chain which will be recorded

Fault is that the fridge and freezer will defrost while still showing the normal temp on the display, so you only find out about it when you open either the fridge or freezer and its defrosted.

This has been going on for the last 18 months and i have had 4 engineer visits (also have to defrost myself every few months)the first two visits sensors were replaced in the back of the fridge the next one was told it needs a new board and it would be ordered next visit he looked at the display pressed a few buttons and said that he couldn’t fix it as no codes were shown.

Anyone any idea on how to proceeed

Thanks

Robert