Why does ice form on the freezer walls and how is it prevented?

There are a number of reasons ice will develop in a freezer.
1. Warm air can seep in through an improperly closed or not sealed door causing the air to freeze on the evaporator coils. This will also happen if the door has been left open too long.
2. An auto defrost fan should be keeping moisture out of the freezer to not cause ice to form, if this is not happening, the fan could be damaged.
3. Typically a self-defrosting freezer will defrost themselves every 6 to 12 hrs., by turning off the compressor and turning on the defrost heater. If any part of the compressor is damaged, the self defrost feature will not work and frost will occur.
4. Low refrigerant levels can cause frost. Low levels can produce warm gas to run over the coils and then freeze.