The most likely problem is a capacitor that has gone bad. The electric motor which compresses the refrigerant flowing through your system requires an extra surge of electricity to start-up. A “starting capacitor” is a small metal cylinder that accumulates a electrical charge, then releases the additional jolt of electricity just as the motor first turns on. If it has gone bad, the humming sound you hear is the motor trying to start, but not succeeding in overcoming its starting inertia. 

    We recommend that you turn off the system to prevent any overheating damage until a service tech arrives and can troubleshoot the circuit to verify if the capacitor—or something else—is the culprit. Right now in our North Florida area a service call with a capacitor replacement will cost
about $200.

    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? 

• 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.