My AC is Making a Clicking Noise but Won’t Start — A Phoenix Tech Explains Why

We know it’s worrisome when you hear a loud clicking noise, and very frustrating when your AC won’t turn on.

Don’t worry, we’ll help you troubleshoot this tricky AC problem.

If your AC won’t start and makes a clicking noise, it means your AC has one of the following issues:

We’ll tell you right off the bat that repairing most of these parts will require professional expertise, but we can explain what the parts are and how they prevent your AC from starting when they are broken.

Sometimes it’s normal to hear the thermostat make a faint clicking noise after you adjust its settings or as it automatically calls for cool air. This noise is caused by a part called a relay, which opens and closes to turn your AC on and off.

If your thermostat is secured correctly to the wall this clicking sound shouldn’t be very loud. But if you hear a loud clicking noise and the AC won’t turn on, it could mean the thermostat is malfunctioning.

What causes a thermostat to malfunction?

• Broken relay
• Frayed or worn wires

How to diagnose a malfunctioning thermostat:

If you only hear clicking near the thermostat (and not near the outdoor AC unit or anywhere else), then the problem is likely with the thermostat.

You’ll need to hire an HVAC contractor to inspect the thermostat and make sure it’s correctly sending signals to the AC.

What the control board does:

The control board receives the command from the thermostat and then sends electrical signals to parts inside the AC (such as the blower), telling them to turn on or off.

Sometimes control boards have a glitchy electrical control that could be causing a relay to timeout and fail, which leads to the clicking sound.

How to diagnose a failing control board:

If you hear the clicking noise near the indoor unit of your AC, the control board could be the issue. A professional will need to inspect the board to find what’s causing the problem.

What a contactor does:

Your contactor sits on the outdoor unit and is basically a big relay that turns the unit on and off.

Your contactor is the “middleman” between the thermostat and your compressor. Once the thermostat sends the command for cooling, your contactor kick-starts the process of starting your compressor.

But if it’s gone bad, it can’t execute the commands sent from your thermostat, and your AC won’t start. So that clicking you hear is the unanswered signal from the thermostat.

How to diagnose a bad contactor:

If your outdoor unit won’t come on and all you hear is a click (but no humming or buzzing), your contactor is likely the problem. An AC pro can inspect the contactor and replace it or repair it if it has gone bad.

Your capacitor is housed inside your outdoor unit and acts like a giant battery that gives your AC compressor the extra voltage it needs to start up.

Fun fact: Your AC probably needs up to 3 times the amount of power at start-up than it uses to run.

Not all ACs have capacitors to help them start up, though. Usually, capacitors are installed on older ACs that need a little extra help to start.

Over time, capacitors lose their charge (just like how batteries lose their charge) and can no longer give your AC the kick-start it needs to turn on. So when the capacitor on an older AC goes bad, the system won’t have the amount of power it needs to start up until the capacitor is replaced.

Learn more about bad capacitors by checking out our blog, “My AC Just Hums and Buzzes But Doesn’t Turn On”.

How to diagnose a bad capacitor:

If you hear a clicking noise followed by a soft hum or buzzing noise but your outdoor unit doesn’t turn on, you probably have a bad capacitor that needs to be replaced. The “buzz” you hear is the motor trying to turn on without help from the capacitor.

Other signs of a bad capacitor:

• Bottom or top of capacitor is bulging
• Visible gooey discharge on the capacitor