My side by side builds up ice in the bottom of the freezer and some in the coils. MSD2436GRW is the model number. Have read some of the other posts and see timer, defrost switch, heater and I also have read something about a mother board that can go bad. Where do you look for the 3 items that are the main culprat of this problem. Also how to you test these items so as not to "parts swap" the problem