Why is my AC humming but not running?

By Dana Whitlock. Updated October 2026. Repair or replace?

That hum is the sound of a motor trying and failing to start. Power is reaching the outdoor unit, but the compressor or fan motor is stuck, and nothing cools. DOE notes that compressor and fan controls wear out, especially on systems that cycle on and off often, and that corroded wires and terminals cause trouble too. Those parts carry high voltage and store charge even with the power off, so your part of this job is short.

Likely causes

A failed start or run capacitor Pro job

Capacitors give motors the push they need to start. When one weakens, the motor hums and stalls. A capacitor can hold a dangerous charge after power is cut, so testing and replacing it is strictly a job for a licensed HVAC technician.

A worn contactor Pro job

The contactor is the switch that sends power to the compressor and fan. Pitted or stuck contacts can leave the unit humming. Worn controls like this are among the common problems DOE lists. This is technician work.

A seized fan motor or debris in the fan Safe to check

Sticks or debris jammed in the fan blade can stall it. With the system off at the thermostat, look down through the top grille. Do not reach in.

A compressor that will not start Pro job

An aging compressor may lock up. That is the most expensive version of this symptom and needs a technician's diagnosis.

The unit's protective delay Safe to check

Many outdoor units have a built-in delay after power returns or the thermostat changes. Rheem's split-system guide says its unit may wait about five minutes before starting. Wait it out before assuming a fault.

Safe things to check first

• Turn cooling off at the thermostat, wait five minutes, then turn it back on to rule out the protective delay.
• Look (from outside the unit) for branches or debris jammed in the fan blades.
• Check the breaker. If it is tripped, reset it once. If it trips again, leave it off.
• Clear plants and debris back at least 2 feet from the unit, as DOE advises.
• If the hum continues, turn the system off at the thermostat so the motor is not left stalled.

When to stop and call

A humming outdoor unit that will not start needs a licensed HVAC technician. Leave the system off in the meantime, since a stalled motor overheats. Do not open the access panel, touch the capacitor, or push the fan with a stick to get it going. If you smell burning or the breaker keeps tripping, NFPA's electrical safety tips call that a reason to bring in a qualified pro.

Repair or replace?

Start with its age. Type the serial from the label:

A capacitor or contactor is one of the cheaper AC repairs, even on an old system. A failed compressor is the expensive one. DOE's analysis puts central AC life at roughly 24 years in northern and hot-dry regions and about 18 years in hot-humid ones. Inspector charts are much shorter, at 7 to 15 years for the system and 8 to 20 for the condenser. Date the system with the age decoder above. When the diagnosis is the compressor on an older unit, the replacement range above is the number to compare.