Our kichen Aid refrigerator is not going into the defrost mode, compressor runs constantly, and after about a week ice builds up causing the fridge side to not be cool enough. fridge side temperature maintains itself at 42 degrees until ice-up. Freezer side works itself to well below zero degrees.

I had the control board replaced, located at the top of the fridge side. This did not fix the problem, so I had the original board reinstalled. I am able to program the unit into test mode to check four functions: evaporator fan, defrost heater, compressor and air door. These all seem to check out ok. Defroster element gets hot. The Air door responds and holds its setting for about a minute then disengages. I am not sure if this is normal.
Reference to the wiring diagram shows defroster cicuitry going to a control box located at the bottom of the unit next to the compressor.

I suspect the freezer thermistor and/or the control panel may be faulty. Is there a diagnostic procedure to check these parts? Am I on the right track?

I ordered and installed a new defrost thermostat-- no change in performance. I appreciate some help in diagnosis so that I may proceed forward with ordering the correct parts.
Thank you,
TR

Gene,
Thanks for the advice on defrosting issue and control board. I am still concerned about the freezer side getting excessively cold. I guess I'll take it one step at a time.
How often and for how long does the defroster cycle run? Is it an electronic timer in the board, or, something more sophisticated?
I'll change the board and hope for the best.
Thanks again for your help.
-TR