Defrost Timer

• If the cooling coil has iced up, it may be because of the defrost timer for the automatic defrost system. This part of the system works by sending a signal to the rest of the defrost components, telling the other parts when to work and for how long. The timer makes sure that adequate time passes to get rid of the frost but not so much that it affects the freezer’s cooling abilities. If the timer malfunctions, it may not signal the defrost system to come on at all, or it could make it stay on too long if it is stuck in the on position. This will require replacement of the part.
  
  

Bi-Metal Switch

• The bi-metal switch is a component of the defrost system that is also a potential problem if the refrigerator is not defrosting normally. The bi-metal switch is basically a defrost thermostat. It monitors the temperature of the freezer and indicates whether the air temperature around the cooling coil is within accepted parameters. When it gets too cold, it indicates the presence of frost and sends a signal to the timer to start the defrost process. If the bi-metal switch is not working properly, the defrost system may not come on at the right time or at all. It will have to be replaced if this is the case.
  
  

Heating Coil

• The key element in the refrigerator’s auto-defrost system is the heating coil. The defrost heater is the part that warms up the area around the cooling coil and causes the frozen condensation from the freezer to melt and drip into a drain below. If the heater goes out, then there is no way for the frost to melt, and the process does not work. Replace the heating coil if it is not coming on and you've ruled out other faulty parts.
  
  

Clogged Drain

• One problem you may be having with the defrost system is condensation drainage. When it is working properly, the defrost system will send the melted frost down a drain in the freezer where it is carried away to a pan beneath the unit to evaporate. But sometimes the water will freeze over the drain and prevent the water from escaping. The results can be a trickle of water into the fresh food compartment below or a sheet of ice on the floor of the freezer. The drain will need to be cleared with warm water to melt the ice or manual defrosting for it to work properly again.