A unit that requires 220 volts of power must have adequate voltage running to the unit, and the same goes for units that run on 110 volts. The voltage can be determined by using a voltage meter.
Our favorite 110 volt air conditioner heater window unit is the Frigidaire FRA08EHT1 8,000 BTU Through-the-Wall Room Air Conditioner with 4,200 BTU Supplemental Heat (115 volts). This is a terrific air conditioner/heater for a great price. Also, you can use it year round, which makes it quite convenient.
A 110 volt air conditioner heater window unit may be checked before and after installing the thermostat. If there is voltage that is being supplied to the thermostat but not from it, then it likely needs to be replaced. It is also important to take a look at the fan motor voltage. This fan runs the condenser fan as well as the indoor blower.
If the motor does not work, the compressor can kick in but it will overheat and turn off before too long. This can eventually result in compressor failure. A larger window unit may have its motor replaced inexpensively, but for a small unit the cost to replace it most likely will be more than if you were to buy a new one.
Central air conditioning units are a bit more complex than window units. There are more issues that may occur with them. The thermostat can cause problems, although there should only be 24 volts going to it. In the case of some units, the voltage can come from the furnace or indoor air handler, while with others it comes from the outdoor unit.
For the most part, central air conditioning is supplied by a furnace or indoor air handler. An air conditioning unit that is only used for cooling normally will have just two wires attached to the condenser unit. Check to be sure that only 24 volts are running across these wires.
Next, you should check the indoor blower. If your thermostat is set to cool the home, then this should be running. If no air is moving across the indoor cooling coil, there soon will be a block of ice on the coil. This can occur if the air filter is clogged, the indoor blower is not working, or if the charge of refrigerant has been lost.
Finally, there could be a total compressor failure. Often times, the compressor will lock up or be unable to turn when there is power being supplied to it. Compressor failure is commonly caused by a lack of lubrication or overheating. It can become overheated when the outdoor coil for the compressor is clogged with grime, dirt or something else.
In order to prevent this, the coil should be washed out every year with water and cleaner. The unit should run at peak efficiency after a cleaning. Changing out the compressor requires a professional with a proper refrigeration license and specialized equipment.
As you can see, there are simple things that a homeowner or a handyman can do to troubleshoot air conditioner issues. Also, specialized tools and an experienced professional may be required for certain things. Be sure that you take precautions and do not conduct such work if you are not qualified.