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If you can remove the back wall of the freezer and examine it, the this will tell something about what is going on.
Heavy icing in one area?
Some icing spread out over the area?
Please let me know what you find. I'll check this post later today again.
Also, can you feel the defrost heater on (around the edges of the freezer where the door meets the cabinet)?
After you have completely defrosted the machine manually (approx. 24 hrs) , you would plug it back in.
If the unit works correct for the next 48 hours then the problem was the defrost system.
At that point, we would definitely replace the defrost timer.