Air Conditioner Not Cooling: 10 Common Causes and How to Fix It

It is a big frustration when a central air conditioner is operating and not cooling a house. The problem of air conditioner not cooling is very common among the U.S. homeowners and in most cases it has a cause. This paper discusses the reasons why a central AC may blow hot air or not cool effectively and how to diagnose each of the reasons.

It is written in simple, third person terms to enable the home owners to know the possible issues and the solutions that they can make themselves and when to call in the experts. The following sections answer a major question regarding AC cooling issues, giving diagnostic hints and solutions.

Why Is My AC Running But Not Cooling?

There are a number of reasons why an AC can run without cooling. The most typical reasons are improper thermostat settings, airflow blockage, dirty coils, low refrigerant or mechanical breakdowns. The system may be running but not cooling in case the thermostat is not set properly or the fan is in the wrong setting.

In many cases, the problem lies in the fact that the airflow is not good enough, i.e., the air filter is clogged or the vents are blocked, and this does not allow the heat exchange to take place properly and even results in the evaporator coil freezing. In other occasions, the outdoor unit (condenser) can be clogged or faulty, and this compromises the capacity of the system to lose heat. Inadequate cooling will also be caused by low levels of refrigerant (Freon) as a result of leakages. Lastly, the AC may blow non-cold air due to electrical or mechanical issues, e.g. a broken compressor or fan motor.

In a nutshell, the table below is a summary of the typical reasons why a central AC is not cooling and the solutions to the problems:

Possible Cause	How to Check or Fix
Thermostat setting or mode issue	Ensure thermostat is on Cool and set below room temperature. Set fan to Auto (not just On). Replace thermostat batteries if the display is blank.
Dirty or clogged air filter	Inspect the HVAC filter and replace it if dirty. Check monthly and change regularly to maintain airflow.
Closed or blocked vents or ducts	Open all supply vents and return grilles. Remove furniture or objects blocking airflow. If certain rooms are still warm, have ducts inspected for leaks or disconnections.
Dirty condenser coil (outdoor unit)	Turn off power and clear debris from the outdoor unit. Gently rinse the outside coil fins with a garden hose to remove dirt. Maintain 1–2 feet of clearance around the unit.
Frozen or dirty evaporator coil	If ice is visible on the indoor coil or refrigerant lines, turn off the AC to let it thaw. After melting, fix airflow issues (e.g. replace filter, open vents). Call an HVAC tech if the coil was very dirty or keeps freezing.
Refrigerant leak / low refrigerant	If cooling has declined over time or the AC runs constantly, refrigerant could be low. Call a licensed technician to find and fix the leak and recharge refrigerant.
Outdoor unit not running	If the indoor fan runs but the outside unit is off, check the circuit breaker or outdoor disconnect. If tripped, reset it once. Otherwise, have a professional diagnose the condenser.
Mechanical failure (compressor/fan)	If the outdoor unit hums or struggles but the fan or compressor doesn’t run, a component may have failed. This requires professional repair or replacement of the faulty part.
Clogged condensate drain line	In humid weather, a clogged AC drain can trigger a safety switch that shuts off cooling. Clear the drain line and flush it with a bleach solution to restore operation.
Extreme heat or undersized AC	In very high outdoor temperatures, even a working AC may not reach the set temperature. Use fans and reduce heat sources, and consider an HVAC evaluation if the system consistently can’t keep up.

All these problems are elaborated in the succeeding paragraphs. These possibilities can be checked systematically, and a homeowner can usually identify the reason why his central air conditioner is not cooling and what should be done about it.

Is the Thermostat Set Correctly?

Whenever the AC is not cooling, always ensure that you check the thermostat settings twice. The thermostat must be set at the cool mode and the temperature must be lower than the room temperature. Check the fan setting, too: when it is set to On all the time, the blower will continue to blow even when the compressor is off, so it may seem that the AC is not cooling.

It is suggested to set it to Auto, and the fan will be switched on only when the AC is actively cooling.

It is also necessary to make sure that the thermostat is powered. In case it is battery powered, ensure that the batteries are not dead (change them in case you are not sure). Ensure that the thermostat screen is up and active.

A no-cooling problem can be solved in a short time by simply adjusting a thermostat that is not set correctly (or replacing a defective one). When the thermostat has been correctly adjusted and is working, and the air conditioner still fails to cool, then other reasons should be sought.

Could a Dirty Air Filter Be the Problem?

One of the most common causes of an AC that is running but not cooling very well is a clogged air filter. The task of the filter is to capture dust and debris, however, when it becomes too dirty, it will clog the airflow. The coil can become too cold and ice up due to lack of enough air passing over the evaporator coil.

Once the coil is frozen in such a manner, the AC is not able to cool the air. The homeowner may experience poor air circulation through the vents or warm air despite the air conditioner being on, which are the indicators of a frozen coil caused by a clogged filter.

The fix is easy: find the HVAC filter (usually behind a return grille or in the furnace/air handler cabinet) and check it. In case the filter appears gray, clogged, or dusty, change it with a new one. It is a simple DIY solution that can make cooling much better.

Always ensure that you use the right size and kind of filter to the system. It is prudent to check the filter once a month in summer and replace it after every 1-3 months when necessary. Cleaning the filter ensures that the air flows well and prevents the evaporator coil to freeze again. A coil that has been iced may take a few minutes to thaw after a very dirty filter has been replaced, and normal cooling restored.

Are the Vents and Ducts Allowing Proper Airflow?

In some cases, the AC is functioning properly but the cool air is not getting to the rooms because of the problems with the vents or ducts of the home. First, make sure that all the supply vents (the registers blowing air) and all the returns are open and clear. One might be tempted to seal vents in rooms that are not in use but sealing too many vents is counterproductive to airflow.

Limited return or supply vents add pressure to the duct system and may decrease the total amount of cool air being produced, in fact, low airflow due to blocked vents may even cause coil icing.

Visit every room and ensure that furniture, rugs or curtains do not cover vents. In addition, leave interior doors open to aid in free circulation of air.

In case some rooms are never cooled well enough or the airflow is very poor, it is possible that the ductwork itself is the problem. Broken or leaky ducts may blow cool air into an attic or crawl space rather than the living space. When there is a suspicion of duct leaks, it is prudent to invite an HVAC expert to check and seal the duct system. Repairing duct problems can substantially enhance the whole cooling performance.

This can be done by ensuring that vents are open and ducts are not damaged to help in ensuring that the cold air generated by the AC reaches where it is required. This move usually solves the problem of uneven cooling and makes the house achieve the desired temperature much faster.

Is the Outdoor Unit (Condenser) Dirty or Blocked?

The other frequent reason of poor cooling is a dirty outdoor condenser coil. The coil of the outdoor unit is in charge of releasing heat of the refrigerant into the external air. When that coil is dirty or when debris is clogging its airflow, the system is not able to dump heat and cooling is impaired.

Leaves, grass clippings and dirt may accumulate on the condenser over time, particularly when the unit is close to foliage or a dryer vent.

A brief look at the condenser may indicate whether it is dirty. Peek through the side grille of the unit – are the thin metal fins covered with dust or lint, or is there some debris such as leaves stuck inside? It should be cleaned. Make sure also that there is plenty of clearance (at least 1-2 feet) around the unit; cut back any bushes or weeds that are too near.

The outdoor condenser unit (above) has a fan that moves air through the coil to dissipate heat. A blocked condenser due to dirt or debris will not be able to cool the refrigerant effectively and this will result in poor cooling. To clean the condenser, turn off power to the AC (turn off the breaker or pull out the disconnect).

With the help of a garden hose, rinse the coil on the outside gently. Do not use high-pressure spray that will bend the fins. A lot of cooling capacity can be regained by merely cleaning off the deposited dirt. Also get rid of any big debris manually (use gloves, if necessary).

In case the coil is very dirty or greasy, a professional coil cleaning can be justified. Cleaning the condenser does not only solve the cooling problem but also avoids straining the compressor in the future.

Could the Evaporator Coil Be Frozen or Dirty?

In case the air conditioner is on and the only air that comes out is warm (or very minimal air), it is possible that the evaporator coil is frozen. When the indoor coil turns into a block of ice, it is not able to absorb heat in the house.

This normally occurs when there is inadequate airflow (through a clogged filter or closed vents) or when there is low refrigerant that leads to unusually low coil temperatures. Very weak airflow out of vents and perhaps visible frost on the copper refrigerant lines at or near the indoor unit or on the coil itself are common indicators of a frozen coil.

The initial one is to inspect ice. When there is a build up of frost or ice, switch off the cooling to allow the coil to thaw. Place the thermostat in the Off position (or simply turn off the entire system) and allow all the ice to melt. To speed the thaw, set the fan switch to On to have the blower move air across the coil. A heavily frozen coil can require a couple of hours to defrost.

After the coil is not iced anymore, deal with the cause. In case it was caused by a dirty filter or closed vents, change the filter and make sure that there is sufficient airflow in the house. Once the airflow is restored, attempt to run the AC once again and check whether it cools.

In the event that the coil freezes up once more even when there is good airflow, then the issue may be low refrigerant or dirty coil. Low refrigerant reduces the pressure and temperature in the coil and it is likely to freeze and it needs a professional to rectify. Poor heat absorption can also be caused by a dirty (on the surface) coil; in this case an HVAC technician may have to clean the evaporator coil during a service call.

To conclude, frozen evaporator coil is a severe indication of a problem, the AC will not cool until the ice melts and the cause of freezing is eliminated. Many homeowners can restore cooling of their AC by defrosting the coil and repairing airflow problems.

Otherwise, probably it is the time to request the assistance of professionals to check the level of refrigerant or to thoroughly clean it.

What If the AC’s Outside Unit Isn’t Running?

In case the indoor blower is running and the outdoor unit is not, the AC will not cool down at all. In this case, the vents will only blow room-temperature air since the compressor and the condenser fan (outside) are not switched on. This may have a couple of explanations:

The first thing to do is to check the power supply to the outdoor unit. Check the electrical panel of the home and check whether the air conditioner condenser circuit breaker has tripped. Assuming that it has, reset it once and check whether the outdoor unit starts up.

A breaker may sometimes be tripped by a storm or overload. Also make sure that the outdoor disconnect switch (a box on the wall close to the unit) is on. It is possible that it is left off by the technician after maintenance or it is accidentally switched off.

In case power is not the problem, then there is a possibility of component failure in the outdoor unit. Typical problems are a bad capacitor or contactor – these are electrical components that may prevent the fan and compressor to engage.

Not much can be done by a homeowner in this situation other than to listen (a quiet humming of the unit may indicate that the motor is attempting to start but is unable to). The other indication is when the condenser fan is not rotating yet the compressor appears to be working (buzzing or humming).

Such failure will have to be diagnosed and repaired by an HVAC professional. The technician may change the capacitor, fix wiring or in worst cases, the fan motor or compressor may require replacement.

In a nutshell, when the outdoor unit does not run at all (not a power problem), it is high time to call a professional. After the problem in the outside unit is fixed and it is operational, the AC is expected to start cooling normally.

Is the System Low on Refrigerant (Freon)?

An AC not cooling may also be caused by a refrigerant leak. The heat carrying fluid is called refrigerant; when it is not sufficient in the system, the cooling capacity is reduced drastically.

The low refrigerant can manifest itself in the form of the AC working continuously and yet only providing a slight cooling effect, or the evaporator coil freezing due to the low pressure. In case the AC was losing its cooling ability with time, then it is probably leaking.

Signs of low refrigerant are a hissing or bubbling noise (indicating a leak) and there may be oily residue on the refrigerant lines or coils (refrigerant oil). Nevertheless, a lot of leaks cannot be easily detected without special equipment.

When the AC is not cooling well despite the elimination of other possible causes, it is wise to have a technician inspect the level of the refrigerant.

The problems with refrigerant should be addressed by a certified HVAC technician. In case the system is low, a technician will find the leak and fix it and recharge the AC with the right quantity of refrigerant, as per the manufacturer specifications. The homeowner should not make an effort to add Freon by himself.

As soon as the refrigerant charge is replenished to normal, the cooling capacity of the air conditioner will be restored. Note: in case an older AC has R-22 Freon, note that this refrigerant is phased out and is extremely expensive; a serious leak may lead to the discussion of an upgrade to a newer system with modern refrigerant.)

Is the AC Unit Too Small or the Weather Too Hot?

Sometimes, the AC may not be faulty at all, the system might be operating correctly but it is not able to satisfy the demand. Central air conditioners are normally designed to cope with a specific maximum outdoor temperature. When a heat wave drives temperatures way beyond normal, AC may be operating all the time and yet fail to achieve the thermostat setting.

As a rule, a central AC can make a house cooler by 15-20 degrees than the outside temperature. When the outdoor temperature is 100°F, the system can only sustain about 80°F in the house. This may be experienced as the AC is not cooling enough yet it is running at full capacity.

Likewise, when the AC unit is undersized to the house (or the insulation of the house is poor, the windows are uncovered, etc.), it might not be able to keep up during very hot days.

To enhance comfort, attempt to minimize the cooling load on the house: close blinds or curtains at the time of the day when the sun is the strongest to minimize heat gain, use ceiling fans to aid in the circulation of cool air, and avoid the use of heat-generating appliances during the day. The following measures will assist the current AC to cool better.

But when poor cooling is a persistent problem, it may be worthwhile to have an HVAC expert to check the system. They will be able to know whether the AC is well sized to the home and whether it is operating efficiently.

In some cases, the performance of the AC can be enhanced by increasing the insulation or sealing of air leaks in the house. Otherwise, in other situations, particularly when the unit is older, it may be the long-term solution to a house that never cools down enough to install a more efficient or higher capacity air conditioner.

What About Mechanical Failures?

Other times the problem is a mechanical failure in the AC system. The system is unable to generate cool air when parts such as compressor, fan motor or control board are faulty. A bad compressor, especially, is a severe problem since it is the one that circulates the refrigerant.

In the absence of sufficient compression, refrigerant flow and pressure drop, there is inefficient cooling.

The abnormal sounds like buzzing, humming, or even no sound at all in case of a component failure are usually signs of mechanical failures. A homeowner may also experience a drop in cooling efficiency with a rise in energy costs.

In such situations, even though a few minor problems (such as a loose fan belt) can be solved by a DIY method, most mechanical problems can only be diagnosed and fixed by a professional HVAC technician.

Many mechanical problems can be prevented by preventative maintenance. A certified technician can also detect worn or failing parts during regular servicing before they cause a total system failure.

Also, by maintaining the AC clean on the inside and outside, one will minimize the wear and tear of the mechanical components and increase the life of the system.

How to Avoid Being Ripped Off and Get a Fair Repair Price

It is not always easy to find the right AC repair expert, some of them may charge the lowest prices as they are new or have bad reputations, and some of them may charge much more than they should do.

That is why the best thing to do to obtain good repairs at a reasonable price is to get estimates of the best-rated local professionals and meet them, get to know about their background and experience of the technicians who will perform the work. Select a well-experienced AC repair contractor at a fair price. You may speed this up by taking advantage of our Free Local Estimates service.

By completing a short form and sending it, you will shortly receive a call of three of the best AC repair and installation companies in your area. They are pre-screened in terms of experience and they are all licensed and insured to protect you. Click below to start and save your home and wallet now.