If your central HVAC system utilizes a combination of electricity for cooling and natural gas for heating, and also has a filter below the bottom front panel of the indoor unit (furnace/air handler), then the problem is likely a small safety switch. It may be in the shape of a pin located at the top of the panel opening that pops out when the bottom panel is removed, like in the photo above; or, it could be a small lever that swings out when the panel opens, as in the photo below. 

    Either way, when the switch is activated it shuts off the system—to prevent the possibility that combustion gasses being created in the furnace directly above the panel opening could be sucked down in the air circulating back into the home below. 

   The somewhat battered yellow sticker above the safety switch in the photo states that “COMPARTMENT DOOR MUST BE SECURED EXCEPT WHEN SERVICING!” Your problem could be that the bottom panel has not been correctly replaced so that it compresses the safety switch, which allows the system to restart. Try resetting that bottom panel, making sure that it is securely pressing against the front metal surfaces of the furnace/air handler cabinet. Also, remember that there is a time delay built into the air conditioning system that keeps the system from restarting for a few minutes.

  If you find that the front panel is securely in place and the air conditioner still won’t start back up, then you have a fault safety switch or some other problem, and need a service call from an air conditioning technician. Occasionally, we come across a system that has had a cantankerous safety switch “fixed” by disabling it with tape to keep it in the closed position, or it has been removed altogether. Both of these are safety defects that will put you at risk for carbon monoxide poisoning. So...don’t do it!

Also, see our blog posts Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting and Why is my air conditioner not cooling enough? 

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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.