An air conditioner covered in ice looks like it is working too well. It is doing the opposite — ice forms when the coil gets too cold because it is not absorbing enough heat, and a frozen coil moves almost no air at all.

The three usual causes

Restricted airflow. A blocked filter, closed vents or a dirty evaporator coil all starve the system of the warm air it needs.

Low refrigerant. Almost always a leak. Refrigerant is not consumed, so if it is low, it went somewhere.

Running it too cold, too long. On a cool night, an air conditioner set very low can drive the coil below freezing.

Never chip ice off a coil. The fins bend, the tubing punctures, and a service call becomes a replacement.

What to do

Switch the cooling off and set the fan to on. That moves room-temperature air across the coil and thaws it, usually in one to three hours. Change the filter while you wait. If it freezes again within a day, the cause is refrigerant or the coil, and both need a technician.