zanussi

Hi.
I will post back with model number later ,but someone might hopefully be able to give advice anyway,on what to check.
Its a zanussi ,large frost free, 6 foot 6……often refered to as a coffin.
Checked for 4 weeks,and ran fine …
Then suddenly temp alarm buzzer started bleeping.
As far as i can remember ,fridge was ok ,freezer running about -8.
Stayed like this ,so turned it off .
There did not seem to be frost build up in freezer,fan and motor was running.
I changed the 2 boards, 1 on the rear,and 1 on front top panel.
Run again for 5 weeks.
ran ok so i sold.
I had checked it was going on defrost.
2 weeks later phone call…..freezer not cold enough.
I will check but call says red light and freezer not cold enough.
Its the zanussi model ,where if freezer door left open for 1 min ,alarm sounds.
Please, does this foult sound familiar.