The moment I notice an AC stopped cooling and won’t turn on is always gut-wrenching.
There are many reasons for an AC not turning on, and I never know if it is a minor sensor tripping or a complete compressor failure.
Living in the South, I know nothing is worse than a broken air conditioner.
Fortunately, it is possible to troubleshoot the reasons why your AC won't turn on with the steps below.
How AC Units Work – Why It Matters When Diagnosing Issues
Before diagnosing an AC unit, it is essential to understand how an air conditioning system works.
A conventional split system uses two main components: the indoor evaporator/air handler and the outdoor condenser.
Knowing which part is causing your issues is the first step in finding a solution.
10 Common Reasons Why Your AC is Not Turning On
Here is my troubleshooting list, which covers the most common reasons for an AC not turning on.
It is based on my troubleshooting steps, from the quickest to check and most common down to the more obscure and hard to test for.
I can’t emphasize enough the importance of diagnosing the problem properly rather than just changing parts based on assumptions.
1. Thermostat Malfunctioning
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.
How To Fix
If the thermostat has no power at all, then it needs replacing. Thermostat replacement is an easy DIY process.
2. Tripped Circuit Breaker
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.
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.
The breaker is sized to the wire or power cord running to the air conditioner.
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.
How To Fix
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.
3. Faulty or Jammed Compressor Motor
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.
How To Fix
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.
4. Failed AC Capacitor or Contactor
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.
If the reading doesn’t match the specs on the side of the capacitor, then it is time for a replacement.
How To Fix
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.
5. Clogged Condensate Drain Line
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.
If you have a clogged condensate drain pipe, the overflow switch will cause a power outage to the air handler.
This will also shut the power off to the thermostat.
How To Fix
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.
6. Faulty Indoor Blower Motor
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.
How To Fix
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.
7. Dirty or Frozen Evaporator Coil
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.
How To Fix
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.
8. Dirty Condenser Coil
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.
How To Fix
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.
I always hose from the inside out, but even hosing from the outside can help remove dirt and debris.
Once cleaned, there should be a noticeable difference in efficiency and indoor temperature.
9. Low on Refrigerant
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.
How To Fix
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.
10. Your AC Has Served Its Time
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.
How To Fix
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.
People Also Ask (FAQs)
How do I force restart my AC?
You can force an air conditioner to restart by turning the breakers off and on in the electrical panel, leaving the breaker in the off position for 30 seconds. Doing so resets both the indoor and outdoor AC systems and the thermostat.
Why is my AC clicking but not turning on?
An audible clicking inside typically means the thermostat is working, but the AC system is not. However, a clicking noise outside indicates that the contactor is engaging. Either case could be a sign of an electrical problem or a failed component that a professional should diagnose.
Why is my AC not cooling, but the fan works?
A working fan indicates that the air handler is on, but the outdoor air conditioning unit is not functioning. When the AC is left unchecked, the current room temperature will rise. Common reasons for this include a tripped breaker, blown fuse, or even a bad start capacitor.