There are many ways to repair an air conditioning thermostat depending on what is wrong with it. Repair may include tripping the breaker, replacing the fuse, checking power switches, checking the compressor and possibly contacting a professional to complete repairs.
The very first step of repairing the thermostat is to check that it is set to the "cool" setting and that its temperature is set below the ambient temperature. This helps distinguish a broken thermostat from one that is simply not blowing air.
Next, check the primary electrical panel for tripped breakers or blown fuses. If necessary, reset the breaker and replace the fuse, and use this opportunity to ensure that the air conditioner, if central air, has a dedicated 240-volt circuit.
The next step is to check the power switch for the furnace and the power switch for the outdoor condenser. Turn on both of these if they are off. Additionally, make sure the 240-volt disconnect located near the compressor has not been shut off.
Finally, turn off the power completely, and check the thermostat. Do this by removing the cover and unscrewing the wire from its Y terminal. Now, turn the power back on and, while holding the wire by insulation only, touch the bare end to the R terminal for approximately two minutes. If this turns on the compressor, the thermostat is faulty and must be replaced. If it does not, contact an air conditioning professional for further assistance.