Ice build up in the evaporator.
Im from Mexico, I had to buy all the parts in US, I made an overhaul to my refrigerator, also I replace the main control board wich has the defrost timer integrated in this mother board, tooks me more de 2 session of 2 hours each one to complete the whole reparation. The most difficult part was remove and install the defrost heater element, because you most handle with the evaporator, need patience and time, also was difficult for me to replace the main control box because the access to it in the bottom of the fridge, too norrow the space and too short the harness cables that connect to it. gasket, defrost control, crisper support and bulb, very easy. excellent service from this site. After almost 3 weeks the service guy I called at the begining is still looking for the parts, Im glad I did my self.