One of the main causes of air conditioner freeze-ups is dirty filters, which reduce airflow and prevent the unit from operating correctly. Once cold air starts building up, it's easy for the unit to freeze and stop working altogether. Shut down the unit and wait for the built-up ice to melt. You may want to place a pan under the conditioner to catch the water as it melts. Take this opportunity to change the filters, which should be replaced monthly. Once the ice has melted out of the conditioner, switch the unit on. Both the compressor — the central box — and the indoor fan should now be running.
If the fan on the inside of the conditioner does not run even after defrosting, the problem is apparent: Without a working fan, cool air can't circulate and becomes trapped inside the unit. This causes the conditioner to freeze up. With the unit turned off and unplugged, check the fans for obstructions, and grease the axle. If the fan still doesn't turn when switched on, contact your repair company, as the fan mechanism is most likely defective and may require replacement.
The thermostat in your air conditioner is designed to withstand harsh conditions, but it has its limits. If the air flowing into the conditioner is too cold, the thermostat may malfunction; consider turning your unit off at night. Check the thermostat by turning the temperature control to a higher temperature than that in the area being cooled by the unit. If you do not hear the two sets of contacts click open, the thermostat is probably broken and may need replacing. If the fan doesn't turn on, the fan relay needs servicing.
The other common cause of air conditioner freeze-ups is low coolant. The coolant is the fluid that allows the unit to maintain such a low temperature, and if it runs out, the fan mechanism will work overtime to compensate and can soon overload. Most units do not allow users to access the coolant tank, so contact a technician to check the levels of your cooling fluid. If the unit is leaking fluid during normal operations or if coolant levels need checking often, the coolant unit may be fractured and in need of repair or replacement.
Finding a Technician
While air conditioner problems can be identified through the troubleshooting steps mentioned in the previous sections, most issues more serious than a simple blocked fan or clogged filter require a trained technician to remedy them. Search your area's yellow pages for a repair person; if you have identified the likely fault, shop around to make sure you're getting a good deal. Make sure your chosen repairer is HVAC-certified, and always get a receipt in case trouble persists.