Most freezer models today have a self-defrost option, i.e., the freezer defrosts periodically without any action on the part of the owner. But if you notice that your freezer keeps defrosting more often than normal, and fails to keep food frozen, it may be due to the following reasons.
1) Incorrect thermostat settings
Sometimes the simplest solution is the answer to the problem. If the freezer keeps defrosting, lower the thermostat. Wait a few hours to see if the defrosting persists.
2) Frost on evaporator coils
Ice on the evaporator prevents the freezer from operating correctly. As a first step, defrost the freezer, following the defrost process described in the appliance manual. If this does not solve the problem repeat the defrost process, but this time keep the freezer off for 2-3 days.
3) Faulty defrost timer
It is possible that the defrost timer has started to fail. An appliance technician can easily test if the defrost timer is faulty. The defrost timer is inexpensive to replace and can be bought from any appliance parts store.
4) Faulty wiring or refrigerant leak
Hire a professional electrician to check for any broken wiring in the freezer system. A refrigerant leak may also cause the freezer defrosting.
5) Bad mechanical part
There could be a problem in the evaporator, condenser or compressor. Once again, it is advisable to have an appliance professional check this. Replacing these parts can be expensive. If the freezer is more than 10 years old, it makes sense to buy a new freezer instead of repairing the existing one. Plus, the new freezer will be more efficient and help lower the energy bills.
If you have a fridge-freezer that is placed outside the home (like the garage), then it may start defrosting in winter. This is because most fridge/freezers have minimum and maximum temperature restrictions for the appliance to operate correctly. When the temperature outside the fridge drops below the minimum working limit, the thermostat shuts off. If you intend to keep the fridge/freezer in the garage buy one that has independent thermostats for the fridge and freezer compartments.