AC Compressor Not Turning On? Here’s What to Do

Click click! Is your AC compressor not turning on?

An AC compressor that doesn’t start up is a bad sign—especially when you need cooling in your home.

In this article, I’ll go over what you should do if your AC compressor isn’t turning on.

I’ll also go over the causes of an AC compressor not turning on and what you can do to address them.

What to do when your AC compressor isn’t turning on

There is a wide range of causes of an AC compressor that isn’t turning on—from simple ones that you can fix yourself, to complex ones where you’ll need a professional.

Here’s a list of steps you should take if your AC compressor isn’t turning on:

  1. Fix the thermostat
  2. Check the fuse or circuit breaker
  3. Change the air filter
  4. Clean the condenser coil
  5. Change the capacitor
  6. Change the contactor

I’ll go over these steps below.

Fix the thermostat

A thermostat that’s in the wrong mode will prevent your AC compressor from turning on.

That’s why I recommend checking your thermostat as one of the first steps for troubleshooting your AC.

If your thermostat isn’t sending the cooling signal to your AC, then your AC compressor won’t turn on.

Ensure that your thermostat is in cool mode and set your setpoint temperature as low as it goes.

If your AC compressor still doesn’t turn on, then read further.

Check the fuse or circuit breaker

If your AC compressor still isn’t turning on, you should check the fuse or circuit breaker.

An AC unit will sometimes blow a fuse or trip its breaker due to a malfunction. The fuse or breaker acts as a safety device to prevent an electrical fire caused by over-amperage of your electrical circuit.

If your AC breaker or fuse trips, you need to figure out what’s causing the trip.

Fuses and breakers don’t trip for no reason. If your fuse blows or breaker trips, there is likely a malfunction in your AC.

Blown fuses and tripped breakers can be caused by something as simple as a bad capacitor, or something serious such as a burnt winding.

Change the air filter

Changing the dirty air filter in your AC will help prevent a low pressure situation in your AC that prevents the compressor from running.

If your AC’s air filter is dirty, then it will absorb less heat in its evaporator coil. The reduced heat transfer from a frozen evaporator causes low refrigerant pressure in your AC system.

In the worst case, a dirty air filter will cause your evaporator coil to freeze up.

As a result of the low refrigerant pressure, the low pressure switch will trip and prevent your compressor from turning on.

The low pressure switch is a device used to protect the compressor. When the low pressure switch trips, the power to the compressor is cut off until the refrigerant pressure returns to a normal level.

So keep your air filter clean by changing it every 1-2 months. Also, ensure that there are no airflow restrictions in your AC system that could cause your coil to freeze up and prevent your compressor from running.

Clean the condenser coil

A dirty condenser coil will give your AC high head pressure and may prevent the compressor from turning on.

So how does a dirty condenser coil prevent the AC compressor from turning on?

A dirty condenser coil will prevent the AC from rejecting heat. This creates high head pressure in the AC condenser and causes the high pressure switch to trip, preventing the compressor from turning on.

When the high pressure switch trips, the power to the compressor is cut off until the refrigerant pressure returns to a normal level.

A dirty condenser coil usually won’t prevent your compressor from turning on. In most situations, the compressor will run for a few minutes, then shut off after pressure builds up in the system. That’s called short cycling and you’ll hear the high pressure switch click and the compressor shut off.

If your AC compressor is short cycling, cleaning its condenser coil may get it running smoothly again.

Change the capacitor

A bad AC capacitor will prevent your compressor from running, so you may need to change your capacitor if your compressor is not turning on.

The capacitor creates a rotating magnetic field that the compressor uses to start up and run.

If your capacitor goes bad, then the compressor won’t turn on.

Oftentimes, a compressor with a bad capacitor will blow fuses or trip breakers—so that’s one symptom of a bad compressor capacitor.

The capacitor used in most AC units is a “dual run” capacitor. The dual run capacitor is actually two capacitors in one—one for the compressor and one for the condenser fan motor.

For more information about AC capacitors, check out my article below:

Change the contactor

A bad contactor will prevent power from going to your compressor and keep it from turning on.

So how does a bad contactor prevent a compressor from turning on?

The contactor is the “switch” that turns your compressor on and off.

If the contactor goes bad, then it won’t be able to switch your compressor on.

One thing that I should mention is that it’s unlikely that a bad contactor is preventing your AC compressor from turning on. In most cases it’s something else that’s the problem. But I thought that I’d include the contactor in my list since it they do go bad from time to time—especially if your unit is old.

There are a few different ways that a contactor can go bad, but one of the most common is pitted contacts.

Pitted contacts are caused by the arcing of electrical current when the contactor switches on and off.

As the contactor turns on and off, a tiny electrical arc forms between the two sides of the contactor. Over time, the electrical arc slowly eats away at the metal and reduces the connection quality of the contactor. 

Eventually, the increased resistance from the pitting of the contacts will prevent the contactor from providing power to the compressor, and the compressor won’t turn on.

Fortunately, the contactor is a fairly cheap part and can be swapped out for a new one if it goes bad.

Causes of an AC compressor not turning on

There are quite a few different causes of an AC compressor not turning on. From simple ones such as a dirty coil to complex ones like a locked up compressor. 

Here’s a list of causes of an AC compressor not turning on:

  1. Dirty condenser coil
  2. Dirty air filter
  3. Lack of airflow
  4. Bad capacitor
  5. Bad contactor
  6. Bad electrical power
  7. Bad thermostat
  8. Bad compressor windings
  9. Locked up compressor
  10. Low pressure trip
  11. High pressure trip

I’ll go over all of these causes, and what you can do to fix them.

Dirty condenser coil

A dirty condenser coil will cause high head pressure in your AC, tripping the high pressure switch and preventing the compressor from turning on.

Clean your condenser coil to prevent your high pressure switch from tripping.

Dirty air filter

A dirty air filter will cause airflow restriction, potentially freezing up your evaporator coil and tripping the low pressure switch, preventing the compressor from turning on.

Change your air filter on a regular basis (I recommend every 1-2 months) to prevent airflow restriction in your AC.

Lack of airflow

Similar to a dirty filter, a lack of airflow from closed vents, air leaks, or other obstructions will potentially freeze up your AC’s evaporator coil and prevent the compressor from turning on.

Keep your air vents clear of obstructions and don’t close your air vents in your home.

Bad capacitor

A bad capacitor will prevent a compressor from turning on since it cannot produce the rotating magnetic field needed for a compressor to start up.

If your AC’s capacitor goes bad, then it will need to be swapped out for a new one.

Bad contactor

The contactor is the “switch” that turns the compressor on. So if the contactor goes bad, the compressor won’t turn on.

If your AC’s contactor goes bad, then you’ll need to swap it out with a new one.

Bad electrical power

Bad electrical power can prevent your AC compressor from turning on. If the power service in your home can’t deliver enough amps, then your compressor will have trouble starting.

If you suspect your electrical power is bad, then you should call an electrician to diagnose the issue.

Bad thermostat

The thermostat engages the compressor’s contactor, so if the thermostat goes bad, then the compressor won’t turn on.

If your thermostat goes bad, then you’ll need to swap it out with a new one.

Bad compressor windings

The windings are the electrical part that makes the compressor spin. If the compressor’s windings short or burn out then the compressor won’t turn on.

If your compressor’s windings go bad, then you’ll need a new compressor. In most cases, it’s usually best to get a whole new condensing unit if your compressor goes bad.

Locked up compressor

An old compressor may lock up after years of wear and tear. If your compressor locks up, it won’t turn on. You may be able to get your compressor to start again by installing a hard start kit—but you’ll only get a few more weeks out of it until it locks up again.

If your compressor locks up, you’ll need a new compressor. In most cases, you’re better off getting a whole new condensing unit if your compressor permanently locks up.

Low pressure trip

If your AC’s refrigerant pressure gets low, then its low pressure switch may trip. If the low pressure switch trips, then the AC compressor won’t turn on. You’ll need to wait for the low pressure switch to reset before the compressor will turn on again.

One of the most common causes of a low pressure switch tripping is low airflow through your air handler. So check if your air filter is dirty or if there are airflow restrictions in your AC system.

High pressure trip

If your AC’s head pressure gets too high, then its high pressure switch may trip. If the high pressure switch trips, then the AC compressor won’t turn on. You’ll need to wait for the high pressure switch to reset before the compressor will turn on again.

One of the most common causes of a high pressure switch tripping is a dirty condenser coil. So clean off your condenser coil if it’s dirty.

About Your HVAC Training Shop Author

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

Leave a Comment