A technician replaced our freezer's thermostat with a thermostat for fridges (I only know this from the box.). It turns on and off alright, and we've got it down to 14C. But the alarm light is on all the time (maybe another problem?) Was he mistaken?
you can't use a fridge stat in a freezer although if its a fridge freezer in one unit then this will only have a single fridge stat in most cases which will control the temp in the freezer as well.
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Try fixfault.com to fix freezers.