Hi there! This HNMF2805 frost free fridge freezer seems to get stuck in the "on" position - so it's all nice and cold, but it doesn't turn off again unless I set the control knob to "0". It will then turn on after a while - and then stay on for hours until I manually turn it off again. I read somewhere that these sometimes need a manual defrost to reduce ice build up - is that what I should do? The machine is 18 months old.

Cheers