I have a fza80 upright frost free hotpoint freezer. It suddenly a couple of days ago lost its freeze. The red light permanently stayed on. The engineer came today took the back off the inside and defrosted it , it started working again. He said there was nothing wrong with it the door was probably left open and it has started frosting up inside. It is happening again. My husband took it off again inside and the side of the coils were all frosted again he defrosted it and works. What is wrong with it please help.

I would use a piece of paper between the door gasket. It should have a good drag when pulling it out with the door closed. Check all around, make sure you have a good seal. If not this, could be the defrost terminator,defrost heater or even defrost timer. But check for good seal first.

Sounds like a defective defrost timer.