I have a Zanussi zx99/5si. Recently the tempreture alarm and light have started to go off repeatedly.

Switching the Fridge Freezer off and back on cures this for 5 or 6 hours.

Switching the entire Fridge Freezer off for 48 hours will cure it for a week or two.

The top Fridge section seems to work fine throughout, but the lower Freezer section gets too warm.

I've been told it could be:-
a) The Defrost Heater
b) The Probe or Tempreture Sensor
c) The PCB

Can you guys recommend how I can figure out which of these things might need changing?