10 Reasons Why Your AC Won't Turn On (How To Fix) (2024)

Written By

Josh Mitchell

Last Updated On

December 20, 2023

The first thing I check is the thermostat. It is easy to tell whether or not the system should be on with most digital thermostats.

Before getting too involved, I always look at the air handler's circuit breaker to ensure it is not tripped. The air handler is the power source for the thermostat.

Finding a circuit breaker tripped could indicate a different and potentially more serious problem.

I like to do a quick test for thermostat issues, which entails turning the system off and on. Most thermostats make an audible clicking noise when they turn on.

After listening for a click, I check to see if the blower motor and condenser kick on.

If nothing turns on, I know the problem is likely the thermostat, and I proceed with the steps below.

• Put the breaker for the indoor unit in the off position to cut the power supply to the thermostat.
• Pop the thermostat off the wall (check for a hidden screw underneath).
• Take a picture of the current wiring.
• Remove the thermostat bracket from the wall.
• Mount the new thermostat bracket.
• Connect the wires to the new thermostat (reference the picture if needed).[1]
• Snap the new thermostat to the wall.
• Turn the power back on and test it out.

A tripped circuit breaker can cause a thermostat to shut off, leading to the air conditioner not turning on.

This can happen to the condenser or air handler breaker, indicating a short or too much current trying to pass through.

While I use the term breaker, remember that the same applies to homes with a fuse box.[2]

However, blown fuses must be replaced rather than simply resetting the breaker.

Fuses and circuit breakers protect a home’s electrical system from an electrical fire.

I know I’m very familiar with breakers tripping when too many appliances are plugged in, but the same can happen with an air conditioning system.

The most common cause is a short inside the compressor.

However, a damaged wire could also cause a circuit breaker to trip, especially if it trips immediately after resetting.

I start by looking at the main wires running to the indoor and outdoor units. These wires are exposed in many houses, leaving them susceptible to rodents.

Additionally, the conduit running to the outdoor unit can break down over the years, providing the wires no protection.

While outside, I look for a surge protector because these devices can fail after a hit from large power surges or frequent power outages.

For damaged wires:

A damaged wire is easily fixed by replacing the wire or adding junction boxes to replace a section.

I determine the best way to repair a wire based on the type of break and where it is.

A broken wire inside the air conditioner unit is easy to replace completely.

However, a break in the main supply wire requires securing the cable with junction boxes on each side of the damage.

Then, I splice a new piece of wire between the two boxes.

For faulty breaker:

A tripping circuit breaker is one of the few parts I will swap out to diagnose.

I start by looking for another breaker that is the same size.

Turning the power off and swapping breakers around only takes a few minutes. If the swapped breaker works, I can purchase another one and swap it out.

As I noted, a bad AC compressor can cause a breaker to trip. This indicates a short inside of the compressor.

A common sign of a compressor problem is that the outdoor unit does not kick on (the fan may run if the breaker isn’t tripped), but the inside blower still runs.

These shorts typically happen when the compressor gets too hot and the coating on the internal windings breaks down.

Once the coating wears off, the bare copper windings can touch, leading to a short.

AC compressors are very resilient and designed to operate under heavy loads.

Compressors have a thermal protection cutoff to prevent damage, which shuts the unit down when it gets too hot.

Many times, I’ve thought that a compressor was dead because it wouldn’t turn on, but it was just overheating.

This is a sign of other underlying AC problems, but I breathe a sigh of relief that it’s not a dead compressor.

To ensure the compressor is not overheating, I take a second to hose it off.

It is easy to tell if there is an overheating problem because steam rises from the hot compressor when wet.

Overheating requires further testing, but there is likely a problem with refrigerant moving through the system or dirt clogging the condenser coils.

A tripped breaker caused by a short inside the compressor is not easily fixable.

Repairing the unit would entail pumping down the refrigerant line, removing the compressor, installing a new compressor, pulling a vacuum, and adding clean refrigerant.

I recommend replacing a compressor only if it is under warranty. Instead, I find that homeowners are better off replacing the condenser or the entire system.

If the outdoor AC won’t turn on, I check to see if the contactor under the service panel is engaged.

I also use a multimeter to verify continuity through the contactor.

Contactors rarely fail, but it is worth checking while inside the unit. The most common part that fails on an air conditioner is the capacitor.

Most air conditioners have two or three capacitors.

Sometimes, a bad capacitor is easy to spot as it will literally explode, leaving a nasty residue.

However, a bulging capacitor is much more common. When I see a bulging capacitor, I know it is time for a replacement, but looks can be deceiving.

Some capacitors look perfect but need replacing. The only way to properly test a capacitor is with a multimeter that can test microfarads.[3]

If the reading doesn’t match the specs on the side of the capacitor, then it is time for a replacement.

If the AC compressor does not turn on, the compressor start or run capacitor is likely the culprit.

In contrast, the fan capacitor is likely bad if the compressor is overheating and the fan doesn’t kick on.

For me, a bad capacitor is a best-case scenario because it is easy and cheap to fix. Any homeowner can do it if they take the proper precautions.

Remember that this involves high voltage and requires closely following the steps below to avoid the risk of shock.

• Pull the disconnect to cut power to the unit.
• Take a picture of the current wiring.
• Discharge the capacitor by touching the wire terminals together with a screwdriver.
• Unscrew the bracket holding the capacitor and slide it out.
• Drop in the new capacitor and secure it.
• Reconnect the wires as they were before.
• Put the access panel back on and snap in the disconnect.

Most local codes and AC unit manufacturers require a cutoff float switch to prevent a clogged condensate drain line from overflowing.

Several recent installations that I’ve seen use multiple sensors.

An AC system has two drains, a primary coming directly from the unit’s condensate pan and a secondary coming from the unit or an overflow drain pan.

A horizontally mounted unit will have a secondary drain pan underneath. This pan should never have water in it.

If there is water, then the main line is backing up, or a condensate pump may be broken.

Fixing a clogged condensate line requires cleaning out the pipe. I usually try to pour a little bleach down the drain line first, as it will kill any algae.

If that doesn’t work, I move on to a canned air clog buster. This requires cutting the line open to get a good seal.

Once the clog is gone, the drain pan may still have water, which must be removed.

Once dried out, the sensor returns to its normal position and lets the AC unit turn back on.

While in the attic looking for any signs of water, I also make sure the blower is running. If the thermostat has no power, the blower will not come on.

However, if the outdoor AC unit is running, but no air is moving, then the air handler’s fan motor is likely to blame.

The fan motor is used to blow cold air throughout the home. A lack of cool air blowing inside is the most obvious sign of a broken central air system.

Fixing a faulty blower motor requires replacing the broken motor inside the indoor unit.

A homeowner can replace the blower motor, but it will take a little while and several tools.

First, I always turn off the electricity because of the risk of shock. I typically use an all-in-one screwdriver to take the air conditioner covers off.

Then, I pull out the blower unit and disconnect the wiring.

Next, I unbolt the squirrel cage from the motor with the nut in the center. After that, all it takes is bolting the new motor in.

The blower assembly slides right back in, and the wiring gets reconnected.

Once the air conditioner is back together, I fire it up and make sure everything works.[4]

On older HVAC systems, a dirty or frozen evaporator coil would not prevent an AC unit from turning on.

However, newer systems with advanced sensors ensure the AC won’t turn on to prevent damage.

I’ve never encountered one of these sensors, but they can be a real pain if they are not programmed properly.

On many air conditioners, a dusty evaporator coil will not prevent the air conditioner from turning on, but it will cause freezing.

This can trip a low-temperature sensor, preventing the unit from turning on.

A dirty air filter will also cause a loss of airflow. You should check air filters regularly, and dirty filters must be replaced.

A clogged air filter leads to poor airflow and the evaporator freezing up.

Many people overlook the evaporator since it should stay clean with a good air filter.

However, I like to inspect them when performing regular maintenance.

A clean coil is critical to keeping an air conditioner working properly.

Cleaning out the evaporator is another job that I recommend homeowners learn to tackle.

The only thing needed is a can of foaming coil cleaner, which is available at big box stores.

For regular cleaning, I just spray them down, gently wipe over them with a soft brush, and rinse them with a pump sprayer.

Keep in mind some coils are a real pain in the butt to access. That is why I can't emphasize enough the importance of using a good air filter.

A good air filter means dust should not reach the condenser coils. However, it is equally important to replace clogged air filters.

Just like the evaporator coils, dirt in the AC condenser unit can lead to serious problems.

The reality is that few of the coils I’ve seen received routine maintenance.

Dirt prevents air flow over the coils, which leads to the condenser and compressor overheating.

Newer air conditioning systems have high-temperature shutoff switches that turn the unit off if the coils get too hot.

The air conditioner blowing warm air or not reaching the desired temperature is a clear single that the condenser is plugged with dirt.

Cleaning the AC condenser is easy, and the main tool I use is a garden hose. Before getting started, I always pull the disconnect for safety purposes.

Then, I will pull the fan cover off to access the inside of the condenser unit.

In rare cases, low refrigerant can be the cause of an AC not turning on.

However, I typically see low refrigerant cause other symptoms, such as coil freezing.

Many air conditioners now come with pressure sensors to detect when no refrigerant is in the lines.

This sensor prevents the outside unit from turning on in case of a complete refrigerant leak.

I also like these sensors because they keep an empty system from running non-stop, which just heats the house and leads to high energy bills.

While homeowners should call a professional HVAC technician to check the refrigerant, you can test the sensor with a multimeter.

The low-pressure cutoff is a normally open switch that closes when the pressure gets too low.

If you believe the air conditioner is low on refrigerant, contact a trusted HVAC professional to check the cooling system.

An HVAC technician can check the refrigerant levels and diagnose the system properly.

An HVAC system has a limited life, which varies greatly depending on how much use it gets.

However, I would start to think about a replacement if anything major goes wrong after ten years.

Minor repairs, like fans and capacitors, are absolutely worth fixing.

Still, it is not worth replacing major components, like the compressor, because the repair bill will likely be close to the cost of a new outside AC unit.

Once a unit reaches the end of its life, the only solution is to replace the air conditioning system.

In the meantime, I would use a window unit to stay cool.

Some people like to save some money by only replacing the condensing unit, but I recommend looking into any rebates or credits that may be available that can help offset a full HVAC system replacement.

The best thing to do is contact a trusted HVAC professional for a second opinion.

They can diagnose what is wrong and offer advice on a replacement AC unit.