Air Flow Problems
If air does not flow freely through the air conditioner's coil, the unit can freeze up. Dirty air filters, obstructed air ducts or a dirty evaporator coil can impede air flow. When this happens, the moisture that the air conditioner has pulled from the air can form frost, rather than draining away as it should.
Lack of Refrigerant
If your central air conditioner doesn't have enough of a refrigerant charge, it can freeze up. Recharging refrigerant is a task that must be done by a qualified repairman certified by the Environmental Protection Agency. Icing can also occur if the metering device fails to provide enough refrigerant to the evaporator coil.
Certified repairman are also required to fix any hose leaks, which can be a cause of air conditioner icing. Ice will form at the site of leaks and continue to grow as the air conditioner is used.