Air conditioning units are designed specifically to cool a residential or commercial space. Many central air conditioning units carry the dual task of supplying heat during the winter. In either case, the enormity of cooling a residential or commercial space causes the unit to draw a lot of power. This is expensive, so most homeowners and office managers set their air conditioning units at a higher temperature when not home or at work. If an inside air conditioning fan unit will not shut off, there are a few remedies.
Turn the thermostat up to a temperature at or near the outside temperature. Wait to see if the air conditioning unit fan eventually shuts off. If it does, the problem is not likely the thermostat. If the unit continues to run, it may be the thermostat. Remove and replace it with a new thermostat.
Check the contactor relay switch. This component "tells" the air conditioning fan to shut off once the thermostat reaches a reading equal to the temperature setting. Turn the air conditioning unit's breaker off at the main electrical panel. Remove the access panel with a screwdriver. Test the contactor with a multimeter to see if it works.
Inspect the duct work. If the duct work has leaks or is obstructed, the air conditioning unit will continue to run because the temperature setting is not being met. Visually inspect the duct work for cracks and listen for obstructions. You can use a spray bottle filled with soapy water to wet the duct work and look for bubbling to find leaks or cracks. Listen to the air flow through the ducts for a whistling sound or a low roar to find potential obstructions.