Programable digital thermostatI was just helping a friend with an air conditioner that wouldn’t start and thought I’d turn our conversation into a blog post that might benefit others.

When an air conditioner won’t start there are a hundred possible reasons, some are simple and some require a professional repairman. We’ll start with some very simple items to check, some of these items were covered on the What To Do Before You Call For AC Service page.


First the simple stuff

    1. First check to make sure the thermostat is set properly. Is it set to A/C or cool? Is the temperature set below the room temperature, so that the thermostat tells the AC to turn on?
Furnace switch shuts off power to the air conditioner as well.

Furnace Switch

  1. Check the furnace switch. This is the big red light switch, (it’s usually red) next to the furnace, or at the top of the stairs if the furnace is in a basement. This switch will turn off power to the furnace/air conditioner. This gets switched accidentally from time to time, especially if you have kids.
  2. Check the door/panel on your furnace to make sure it’s fully shut. If you just changed the filter or took a look inside your air handler you may not have shut the door fully. Some air conditioners have a safety shut off switch on the door that shuts off the blower if the door is opened.
  3. Check the breaker. Make sure nothing has tripped (shut off) the breaker for the AC or furnace. If it has not been tripped turn it off then back on in case it tripped and the switch doesn’t look like it.


OK that takes care of the simple things that should be checked before anything else, now let’s get to the thermostat and see if this is the cause of your problem.  First I’m assuming you have a programmable thermostat. If not go out and get one the money they save in cooling and heating costs make them one of the best money saving improvements you can do to your home.

Remember each time you adjust a thermostat setting your AC unit may take some time to apply the changes; this is to protect your furnace and air conditioner. So wait a minute or so between changes to see if the unit turns on or off.


Now it’s Thermostat Time

  1. Check the batteries, or just go ahead and change them. Especially if you just said to yourself “the thermostat has batteries”?
  2. With the thermostat set to off make sure the temperature is set low enough to turn on the AC.
  3. Now set the thermostat to cool if either the outside condenser or the inside blower comes on the thermostat is working. If only one of the units come on that gives you a good idea where the problem lies. If neither unit comes on the problem could be your thermostat or your thermostat wires.


There are two ways to check if your thermostat is working. The first way is safest but requires another thermostat. The second can be done without an additional thermostat but if performed incorrectly could cause further damage to your AC unit. Please watch the video below for the instructions and to see if you feel comfortable performing the procedure.


  1. The first way to figure out if the problem is with your thermostat or your wiring you need another working thermostat. First replace the old thermostat with the new spare one that you are sure is working correctly. When doing this use care to make sure the wires are connected the same way they were on the old unit. If the AC turns on the problem is with your thermostat and should be replaced. If the AC does not turn on the problem could be your thermostat wires or your air conditioner.
  2. The second way to check if the thermostat is working is by connecting the different colored thermostat wires to each other in a specific order. But before you do watch the video and read the warning below.


First remove the cover from the thermostat and find the colored thermostat wires. There should be at least four, white, red, green and yellow. The red wire is the power wire, the green is for the fan, the white is the heating wire and the yellow is the air conditioning wire. Before you do any testing make sure each wire is connected to the thermostat at the proper terminal for example, Y for yellow, R for red, G for green and W for white. If the colors of the wires do not match the correct terminal do not proceed with this test. To test if the thermostat is not working you have to remove the red, green and yellow wires from their terminals. Then connect the red and the green wire together and the fan should go on. When you connect the yellow wire to the other two the outside condenser unit should go on. If nothing turned on the problem is not the thermostat, but it could be the wiring that runs to the air handler from the thermostat or something else with the air conditioner.

Warning wiring your thermostat incorrectly can damage your thermostat and your air conditioner. Proceed at your own risk. This website is not responsible for any damages caused by following the actions shown here.

Like I said at the beginning of this article there are countless things that could cause your air conditioning system to not work, but hopefully this article helped to eliminate one. Remember I’m not a professional air conditioning contractor, I’m just an average home DIYer who tries to figure every problem out before I call for help. And again I hope this is helpful to someone.

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