Samsung RS21 Giving F-DEF ERROR

Hi all,
I have a Samsung RS21DCNS giving F-DEF ERROR.
It was suffering the usual frosting around the evaporator element and I replaced the defrosting element and thermistor part number DA32-00006W and checked the thermal fuse.
A few weeks later it was frosted again and I had noticed on the previous strip down that the foam seals were breaking down on the plastic cover. I therefore fitted a new cover DA9705290Q to ensure good cold air circulation and also replaced the drain heater whilst I had it stripped down again.
To my annoyance after around 4 weeks it’s frosted again. Checking the diagnostics shows F-DEF ERROR and diagnostic force fridge defrosting sounds likes it’s working but thought I would hold off before seeking input and advise from helpful forum members here.

Have I been unlucky and got a faulty new sensor DA32-00006W or is it possible that it could be something else I’m missing ?

Any tips or help to identify what to check next or how to accurately fault find as the SM and diagnostics are quite general
[h=1][/h]