Appliance Repair Questions? Ask a Technician for Answers ASAP
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It sounds like the thermostat is faulty if its reading -18 but that isnt really the case, before looking to replace any parts its always worth defrosting the freezer over 24 hours, as often you get a build up of ice behind the rear compartment (which you cant see).
This costs relativity nothing, and fixes 33% of these type of issues, so its worth doing before you look at testing the thermostat and control board for a fault.