When you a/c system won’t come on you could have a serious problem requiring service by an air conditioning technician, or it could be something simple that you can fix yourself. Here’s a list of things to check first before calling a service contractor: 

1) Check the thermostat to make sure it is adjusted properly. 

It should be set at “COOL” and “AUTO” with the temperature setting below the current air temperature. If your thermostat is battery-powered, a low-battery symbol or no screen image indicates that weak or dead batteries are the problem.

2) Check the disconnects for the system. 

One of the more common reasons we find for an a/c system not functioning, especially where someone has moved into a home recently, is that they are not familiar with the location of the disconnect at the indoor unit, called the air handler. The air handler is often in the garage with the disconnect on the wall next to it. Some disconnects are identical to a light switch—and get shut off by accident. Locate your indoor disconnect and make sure it is on. It may also be a breaker switch or pull disconnect inside a small box.

    Next, check the disconnect at the outdoor unit, called a condenser. To verify that a pull disconnect is supplying power to the condenser look for the “ON” lettering right-side-up at the top of the pull piece. If “OFF” is at the top and “ON” is upside-down, then power is disconnected.And, third, check the breaker in your electric panel to see if is tripped. A tripped breaker switch will be offset from the other breakers in the row and you will see a red box next to the switch in some panels to highlight that the breaker is tripped. If you reset the breaker and it immediately trips again, stop and call for service. 

3) Your condenser could have a safety device that locks it off after sensing certain malfunctions. 

Reset it by shutting off the power at the breaker for the condenser in the electric panel. Wait about a minute, then turn the breaker back on. Because there is an built-in automatic time delay, it will take up to 10 minutes for the system to turn back on. If is shuts off again shortly after restarting, stop and call for service.

4) If there is a “RESET” button at the condenser, press it. 

If it shuts off again shortly after restarting, stop and call for service.

5) If there is a condensate drainage problem, then a safety float switch may have shut off the system. 

If you see a gadget like the one pictured below on the condensate water drain line near the air handler, pull the top and see if there is standing water in it.

    There is also another type switch that is clear plastic, to so you can easily see if there is any standing water in the condensate drain line, also shown above.

    When your air handler is in an attic or other location where spilled condensate water would be a problem, then there will be a pan under the air handler, also with a float switch on the edge flange. If there is standing water in the pan, that means that your primary condensate drain piping is clogged and the drain from the catch is also clogged. Flushing out a condensate drain line is easy if there is a port built-in for it. If not, you may want to have an a/c service contractor do it for you.

    If you have a condensate pump (a small box with a pump on top, usually on the floor under the air handler), check to see if the holding tank is full. There is also a sensor here that would shut off the system. Since these are usually plugged into a 120V receptacle, also make sure it hasn’t come unplugged.

6) Check to see that the bottom front panel of the furnace/air handler is snugly in place.

If you have recently removed the bottom panel of your furnace/air handler to change the air filter behind it, there is a safety switch along the top of the panel opening that shuts off the system when the panel has been removed or is loose. Make sure that panel is securely in place so that the switch is not activated. This only applies to systems where the bottom front panel of the air handler must be removed for a filter change or any other routine maintenance. The switch can be a lever-type, like the one shown below, or a pop-out button.

    And if all these checkpoints turn up nothing, yes...stop and call for service. Also, see our blog post Why is my air conditioner not cooling enough?

Also see Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting 

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while?  

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.