Ice builds up on the wall have to defrost one a week this pass week defrost for two days plugged back up so far its fine but will not give ice is it the mother board are do I need a new one

You have a failure in the defrost system. To diagnose it you need to thoroughly defrost the freezer and remove panel covering coils in freezer after unplugging unit. Use an ohm meter to test the defrost heater. Sometimes you can see if the heater is burned in two. There is a defrost t-stat that is attached to one of the wires that supplies power to the defrost heater. Unplug it and carefully remove it from the coil. Be very careful as the coil can easily be punctured or the capillary tube supplying freon to coil can be broken if you pull on it or bend it too sharply. This will ruin your refrigerator, so be very careful. Take terminator and freeze in another freezer and use an ohm meter to test it for continuity. The switch inside should close at about 32°F. Sometimes there are three-wire switches where one wire feeds the fan in the freezer. That switch closes at about 20°F. Make sure to test the fan to see if it works by testing continuity unless you know it works. If heater is burned out you need to replace it very carefully and it is advisable to replace defrost terminator as well since it usually goes bad about the same time as the heater. I do not know what kind of timer you have, but if it is an electronic refrigerator it is in the electronic circuit board and failure is rare. Hope this helps. DrTom4444