KitchenAid French door refrigerator, model KBFA25ERSS01.

Our normal temp setting are 38 degrees refrigerator, 0 degrees freezer.

About a month ago I noticed the freezer was not recovering very quickly (fridge seems to return to 38 fairly quickly), and sometimes the freezer is back to 0 in the morning but other times is only back to 3 to 5 degrees after running all night, and during the day, looses ground, sometimes up to 10 degrees.

I first cleaned lint and dustbunnies from the back fan area, behind front lower removable bezel and through that, the underneath coils as best I could.

I noticed that the thermistor cover on the back wall, upper right side of the freezer compartment seems to ice up to where the ice is protracting through the slots in the cover. (I have made sure there are no food packages that would touch this and the area around it when door is closed is clear.) I opened the cover and found the thermistor was completely imbedded in ice, so removed the ice and replaced the housing.

Since I have done that, the freezer temp has been about 14, 15 or 16 (setting is on 0) for about 2 days, and I have not noticed any more ice forming on the thermistor area, but the freezer temp is not coming back below 14. (Fridge compartment is cooling normal at 38)

Do I have a bad thermistor, or something worse?