What are the signs of a failing AC capacitor?

Quick Answer

A capacitor gives the AC's motors the push they need to start, then helps them run smoothly. When one weakens, you may hear humming while the fan or the compressor. The pump in the outdoor unit — will not start. You may notice hard or uneven starts. Lose cooling suddenly on a hot afternoon. Only an electric test confirms it.

What you can safely check

• Watch from outside the cabinet. Does the outdoor fan start when cooling is called for? Does the unit hum without starting? Note what you see and hear for the technician.
• If the unit hums without starting, switch it off at the thermostat so the motors stop straining, then call.
• Never open the outdoor unit to look at a capacitor. A capacitor stores an electric charge even after the power is switched off. That stored charge can cause serious injury.
• Never nudge a still fan blade with a stick to "help it start." The motor can catch without warning. Spinning the blade proves nothing.

What might be causing this

A capacitor is an electric part that stores a charge and helps a motor start and run. It is small and shaped like a can. Motors need more power in the instant they start than while they run. The capacitor stores up a burst of electric energy and releases it to supply that extra push, then keeps helping the motor run.

Capacitors work hard on every cycle. TExas heat is rough on them. Heat is what wears them out. They are one of the most common summer failures. They rarely fail in a dramatic way. They fade.

The signs are in how the system behaves:

• The outdoor unit hums, but the fan does not spin. The motor has power and is straining, but the starting push is too weak.
• The system starts hard, or starts sometimes and not others.
• Cooling quits on the hottest afternoon of the year. Exactly when a weak capacitor faces its heaviest load.

Here is the trap: a failing motor and low voltage can produce nearly the same signs. If you replace the capacitor on a guess, you may pay for a good part while the real fault keeps damaging the system. A technician tests the capacitor with a meter and compares the reading to the rating printed on its label. That test is what separates a diagnosis from a hunch.

When to call a professional

Call if the unit hums without starting, starts hard or only sometimes, or stops cooling suddenly. This repair is pro-only. The reason is safety, not salesmanship: the part holds a dangerous charge even with the power off. The technician will test the capacitor, check the motors and voltage. Confirm the real fault before replacing anything.

What happens next

On a diagnostic visit, the technician tests the capacitor and the parts that imitate it. You receive a written diagnostic report and a written repair estimate. No repair is performed until you approve it. Your notes — what hums, what starts, what does not — shorten the visit, so write them down before you call.

Texas Heat & Air next step

Humming with no start is a capacitor suspect until tested. Call (682) 438-8127 and describe the behavior. What hums, what starts, and what does not.

Related Questions

Why is my AC making a buzzing or humming sound?Why has my outdoor AC fan stopped spinning?What happens when an AC contactor fails?

Still Have Questions? Ask an HVAC Expert.

Couldn't find the answer you need? Tell us what's happening. Our team will review your question and contact you to help.

Ask Your Question →

Sources