Not Defrosting

Hi, help needed. The Kitchen Aid refrigerator I am dealing with has a an issue with a defrost. Evaporator coils in the freezer are iced up. I defrosted the freezer manually and checked the heater, bi-metal and evaporator fan. All this elements work fine. Heater has a 30 ohms which is good. Bi-metal passed a continuity test. Evaporator Fan runs well.
The only suspect at this moment is a control board. This unit has few control boards. Please advise which one is responsible for a defrost. The part # would be helpful not just a general main control board indication. As well if anyone has a files on how to run a self diagnostic test on this fridge that would be great. If anyone had a defrost trouble on this unit in the freezer compartment please advise. Thank you