I just bought a boat that has a separate fridge and freezer by sub-zero. The fridge works fine with a low of 34 degree. The freezer won’t get cold. inside the freezer I removed the cover showing the fins. The very small thin tube gets about 6 inches of frost on it but that is all. The unit is about 15 years old had not been used for at least a year.
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It appears it may either have a blockage in the sealed system, or it simply needs a charge of refrigerant.