A digital thermostat can be powered by either batteries or the house electrical system. Pull the front plate of the thermostat and look for batteries behind it. They are usually AA or AAA batteries, and replace them to see if that fixes the problem. Also, check the connection points for corrosion because, if old batteries have been left in the thermostat, they might have corroded the connections to the point where they will not make a good contact with new batteries. Sanding off the rust with a nail file may help.

    If there are no batteries in the thermostat, then it is not getting power through the electrical wiring of the HVAC system. Check the circuit breakers at the electric panel, and also the disconnect boxes at both the indoor and outdoor units. It is not unusual at foreclosed or abandoned houses for the the pull-disconnect plug to be missing.

    Still no luck? It may be a thermostat wiring connection problem, wiring damaged by a grass trimmer near the outdoor unit, or the thermostat may have simply gone bad and needs replacement. These possibilities need to be checked by a technician, and further evaluation by a licensed HVAC contractor is the next step.

    Oops, almost forgot! There is one more possible solution if you have a combination gas furnace and electric air conditioner, and have recently replaced the air filter. Did you have to remove the bottom panel of your furnace/air handler to change the filter behind it? There is a safety switch along the top of the panel opening that shuts off both the system and the digital thermostat when the panel has been removed or is loose. Check to make sure that the panel is securely back in place so that the switch is not activated.  The switch can be a lever-type, like the one shown below, or a pop-out button.

    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 long does a home thermostat last?

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

• Why is my air conditioner not cooling enough? 

• 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.