Frost free refrigerators have an auto defrost cycle that has a heating element that warms the evaporator coil for 15-30 minutes every so often to melt the frost that builds up on the coils. It runs on a timer. If the defrost cycle is not working it could be either the heater or the timer that is bad. A refrigerator repairman would have to diagnose the situation
It sounds like you have a defrost system problem: When a defrost problem occurs, it is usually the fault of one or more of three basic parts:
(1) defrost timer/adaptive defrost control (depending on your model),
(2) defrost freezer thermostat or
(3) the defrost heater
A/c unit
