Why Is My AC Unit Buzzing but the Fan’s Not Spinning?

September 29, 2018

Oh, no—your air conditioner’s not working. You go outside to check the A/C unit, and it’s buzzing but the fan’s not spinning. What’s going on?

You may have a bad capacitor. The good news is, it’s pretty easy to find out. A bad capacitor is a common A/C problem with a straightforward fix.

First, what’s a capacitor and what does it do?

A capacitor is a small, cylindrical object that stores a lot of energy—similar to a battery. Capacitors send an electrical jolt to start a motor (a “start capacitor”) or to keep one running (a “run capacitor.”) In an air conditioning system, capacitors work with three different motors: the compressor, the indoor blower fan, and outdoor fan.

It takes much more energy to get the outdoor fan going than it does to keep it running. It needs that extra boost from a start capacitor. So, if the capacitor attached to your fan motor is bad, the fan won’t start spinning.

Here’s an easy way to test if the fan’s capacitor is bad.

Slide a long, thin wooden stick through the fan grate and gently push one of the blades to try to get the fan spinning. (Safety note: DO NOT use any kind of conductive metal or material to do this. Don’t put your fingers in or near the grate.)

If the fan takes off and then keeps going on its own, you more than likely have a bad start capacitor. You took over its job of kick-starting the fan by pushing the blades.

If the fan doesn’t start running, you may have a bad fan motor. If you have difficulty getting the blades to turn, there may be debris wrapped around the fan axle, or the motor bearings may be dirty or jammed.

Here’s a second way to check for a bad capacitor.

Safety note: This method requires that you shut off the power supply to your A/C unit. DO NOT try the following unless you’re completely comfortable and familiar with that process. You risk dangerous electric shocks if there is still electric current flowing to the unit.

Capacitors store power and may have the ability to send electric jolts even with the power to the A/C unit shut off. DO NOT touch the capacitor terminals or you may get shocked!

Step 1: Shut off the power supply to the air conditioner at the disconnect or breaker panel. It’s located on the side of the building near the outdoor A/C unit. DO NOT proceed if you’re not sure you’ve properly shut off the power.

Step 2: Remove the service panel on the A/C unit.

Step 3: Find the start capacitor for the fan. It’s a small, cylinder-shaped or oval metallic object with two or three prongs on the top for wire connections.

Step 4: Inspect the condition of the capacitor. While not always present, common signs of a bad capacitor include a swollen top, leaking fluid, and/or corrosion or rust on the bottom. DO NOT touch the capacitor terminals or any leaking fluid.

Okay, my capacitor is bad. Now what?

Turn off the A/C unit and call in an HVAC technician to replace the capacitor as soon as possible. This is a quick and simple repair for a professional.

If you leave the unit running, the fan motor will keep attempting and failing to start. The capacitor may also be used to keep the fan motor running properly as well as starting it. (This is called a “start/run capacitor” and they’re used in many A/C systems.) In either case, a bad capacitor will strain the fan motor and may cause it to burn out completely.

Depending on your A/C unit, the same capacitor may be used for both the fan motor and compressor. This is called a “dual capacitor” and has three terminals on the top—one for the fan motor, one for the compressor, and one shared. Your compressor can overheat if it continually attempts to access a failed capacitor, causing major damage.

Don’t risk a costly fan motor or compressor repair by putting off a simple capacitor replacement!

Can I replace the capacitor myself?

It’s possible, but not recommended unless you are completely comfortable with and have experience working with the electrical components of your A/C unit. It involves dealing with high voltage, and there is a risk for serious or even deadly electric shock if not done safely and properly.

The power to the A/C unit must be shut down before opening the service panel, and the capacitor itself discharged of stored energy before it can be removed. Otherwise, touching two capacitor terminals at the same time will cause an electric shock. Disposing of a capacitor that hasn’t been discharged can even cause a fire.

After the bad capacitor has been safely removed, replacement is a matter of selecting the right replacement and attaching the wires correctly.

What causes a capacitor to fail?

Like batteries, capacitors lose the ability to hold a charge over time. They can also suffer damage from high temperatures or high voltage. Scorching outdoor temps plus the heat of the motor can overwhelm a capacitor and cause it to fail. Regularly scheduled A/C maintenance can help spot potential capacitor issues before you’re left without A/C on a hot day.

In the Wichita area, give us a call at (316) 462-2572 for help with your A/C problem. We offer 24/7 emergency HVAC service, free estimates on new installations and in many cases, same-day service. We work on all major brands of HVAC systems.

We do our best to provide current and accurate information, but this content could contain errors or information that is not correct for your situation or equipment. Resources found on our website are provided as general information. Reddi Industries does not assume any liability resulting from the provided information. If you attempt to repair or modify HVAC, plumbing, electrical, or other equipment in your home or business, always consult your equipment’s operating manual first, and only do so if you are qualified.