Filters

• One of the main causes for an air conditioner to freeze up is the condition of the filter. If the filter plugs up with dirt and debris, it can't pass the proper airflow across the coils. This causes the Freon to become too cool, and fit reezes the coils. Since Freon is meant to freeze in its normal state, the air conditioner design keeps air flowing over the coils to inhibit freezing.
  
  

Refrigerant Level

• Because the modern air conditioner is designed on a delicate balance of different factors, the level, or "charge," of the refrigerant will affect whether the system operates properly. If the refrigerant charge is too low, it can result in either poor cooling capability, or simply freezing up the coils. This is because there is more room inside the coils after the Freon evaporates, letting the Freon expand more as a gas, and thus, freeze the coils as the temperature of the gaseous Freon becomes too cold.
  
  

Thermostat Malfunction

• Every air conditioning unit is controlled by a thermostat, be it on the wall somewhere in the home, or on the unit itself. This is where you tell the air conditioner how cool you want the room. The thermostat then measures the inside temperature, and tells the air conditioner whether to keep cooling, or if it is satisfied. If the thermostat malfunctions, it may not tell the air conditioner when to turn off the compressor, which can in turn freeze the coils.
  
  

Fan Motor Failure

• As explained earlier, air flow is very important for the proper operation of an air conditioner. Most air conditioners utilize two fans, one blowing the "cooled" air back inside and one blowing the warm air outside. If either malfunctions, it can and likely will result in improper air flow, causing the coils to freeze. This happens because, as the Freon can be allowed to get too cool, it must rise to a certain point. If it doesn't, it is cooler than normal going into the evaporation process, allowing it to reach a lower temperature with each pass.