AC Unit Humming But Not Turning On? [Causes + Solutions]

Liz Yang

On hot summer days, an air conditioner is a game changer. They are not appliances that break easily, but they can become faulty at some point. It is not uncommon to see an AC hum or buzz but not turn on, but most people have no clue about its causes and solutions. 

In most cases, a malfunctioning capacitor or contactor is the main culprit of an AC humming but not turning on. 

However, many of us don’t know what these two parts are, let alone how to fix them. Don’t worry! I have got you covered! In this post, I’ll answer these questions thoroughly so you can decide whether you can fix it yourself or hire an HVAC technician.

Why is your AC unit humming but not turning on?

There are plenty of causes of an AC making a humming noise, but under some circumstances, the AC can continue to work, even if it produces an annoying humming.

If you find the outdoor unit or indoor unit is humming but won’t start, you should look into the condition of the capacitor and contactor.

A defective capacitor 

faulty capacitor

The capacitor functions as a battery for your air conditioner. It stores some electricity in order to provide a burst of energy when the compressor or fan motor wants to start. As you may know, these parts require far more energy to start than they do to run.

It appears to be a small cylinder-shaped metallic object located on the side of the outdoor unit and connected by numerous wirings.

A high-quality AC capacitor can typically last more than ten years, but it will eventually wear out, just like any battery. When the capacitor is unable to provide enough energy for the fan motor to kick in, your air conditioner will make a humming sound but will not run.

That is, the condenser fan will not spin, and the compressor will turn off.

Furthermore, when the capacitor of the indoor blower fails, the indoor AC unit may hum but not turn on. However, it is more common in outdoor units.

Some indications that a capacitor is failing include an increase in the electricity bill or fluid leaking from the battery case. If you are unsure whether the capacitor has failed, look for any of these symptoms.

A faulty contactor

faulty contactor

Another reason for an outdoor AC unit humming but not turning on is this. However, not many people have mentioned it.

The contactor is responsible for delivering power to the outdoor unit. When it receives a signal from the thermostat, it lowers the inside relay switch, allowing electricity to flow through.

Similar to the capacitor, it is located beside the compressor. In fact, these two parts stay together. It is a black rectangle object positioned vertically.

When it fails, the outdoor unit will not receive the power to start but can still continue to hum or buzz.

Unlike a capacitor, it is hard to tell its condition from its appearance. You need to disconnect it from the condenser unit and use a multimeter to test if there is any faulty coil connection.

How to repair an AC that is humming or buzzing but not running?

When you come across this situation, please calm down. It is not the end of the world, and you may even be able to fix it yourself if you have some related experience.

A quick fix 

When the root cause is a bad fan capacitor, there is a short-term fix if you can’t get a new capacitor immediately.

Because the motor cannot spin the fan automatically, you must manually push it. Push the blades with a screwdriver or similar tool, and it should begin to rotate normally.

Please exercise caution when doing that because it may hurt you because the power is on.

Nevertheless, if the contactor is to blame, there is no quick fix.

An ultimate solution

how to fix ac humming but not turning on

Your best bet is to replace the part regardless of which part can’t function well. Luckily, these parts are affordable. The price of a contactor ranges from $10 to $50, whereas a capacitor will cost you around $10 to $100. On average, you can get both parts at below $50. 

If you want to replace them on your own, you should take a picture before taking apart the old ones. This image could be a great reference later on.

Meanwhile, you should turn off the breaker and power supply before getting your hands on them. It is also important to get a new one that fits your AC. An improper one can damage your machine further. You can get the information from the manufacturer. 

It is not a task that is too complicated to finish. Removing the old one and connecting the wires to the new one will do the trick. Nonetheless, it is only recommended to people who have the experience. 

Otherwise, you should make a call to the local contractor. The HVAC professional will install it for you. However, it will cost you an extra few hundred dollars. On average, the cost will be around $250. They will install it within an hour in most cases.

Sum up

When dealing with an AC humming or buzzing but not turning on, you should first check the capacitor and contactor located inside the outdoor unit. 

Since a bad capacitor will make the fan motor unable to spin and a broken contactor will cut off the power supply of the outdoor unit, they are most likely causes of an AC humming but not starting.

Fortunately, these parts are inexpensive and simple to replace. However, you should only do so if you have relevant experience and take appropriate safety precautions.

Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested a lot of products in person, like humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.