If you are getting a frost build up on the back wall of the freezer, you have one of two problems, that I know you can repair yourself easy. It will either be the motherboard on the back side of the refrigerator, in a panel by itself, or the drfost heater, located on the frozen evasporator coil on the bottom of it. What you need to do is unplug the refrigerator, and take the back panel off the freezer, and take the 2 phillips head screws off the heater housing, and then pull the wires off the heater, and ohm it out. If it is good, replace the motherboard. If you don't have a meter get a cheap one. Tom ApplianceEducator.com