How to troubleshoot a central AC that runs but doesn’t cool

An underperforming AC unit can make your home feel stuffy and leave you sweating, despite the system seemingly working. We’re here to help you pinpoint the cause so you can get your central air conditioner running and enjoy the cool, refreshing air it was designed to provide.

If you find out that you need any replacement parts for your appliance while you’re troubleshooting, you can enter your model number at AppliancePartsPros.com to order them. Most orders arrive in just two business days, and we have thousands of free guides to show you how to install your new parts.

The troubleshooting information provided in this article may not apply to your specific appliance model. While we strive to offer helpful advice for DIY appliance repairs, variations in models and configurations can lead to differences in troubleshooting steps, error codes and part replacements. For the most accurate guidance, we strongly recommend consulting the technical sheet for your specific appliance model.

Safety precautions

When working on any appliance, remember to keep safety first. Here are some tips to keep in mind: 

• Always power off and unplug your appliance or switch off the circuit breaker before attempting any maintenance or replacement work. This keeps you safe by preventing any risk of injury from electric shock.
• Wear insulated work gloves to protect your hands from sharp metal parts, pinching hazards and debris.
• Take your time and don’t rush while working to prevent accidents and personal injuries.
• Work in a well-lit area so you can clearly see and access the interior parts.
• Clear your workspace of clutter and other obstacles. Keep children and pets away from the work area.
• Never work on internal parts with bare wet hands. Make sure the work area is completely dry.
• Check your user manual to see if there are specific installation or safety instructions for your part or appliance. 
• Be gentle when handling or removing parts. Excessive force might damage the appliance or cause injury.
• Wear safety glasses when working with chemicals, dust or cleaning large debris to prevent injury. 
• If the appliance has recently been used, give it plenty of time for any heating parts to cool down before working on it. 
• Take pictures or make a note of wiring terminals or other connections before disconnecting to prevent any problems with reassembly.
• When working with wires, avoid touching any exposed wires or terminals. If you need to touch a wire, use a non-conductive tool or wear insulating gloves to prevent electrical shock.

Tools and materials needed

• Multimeter 
• Refrigerant leak detector 
• Coil cleaning brush 
• Vacuum

The specific tools required may vary depending on the model of your appliance and the type of fasteners used on it. Always consult the user manual or refer to any specific instructions provided by the manufacturer for the recommended tools for your appliance model.

Possible causes and troubleshooting steps

Blocked or dirty air filters:

1. Remove and inspect the air filters. If they are excessively dirty or clogged, replace them with new filters.
2. Clean or replace the filters as recommended by the manufacturer, usually every 1-3 months during periods of heavy use.
3. Ensure the filters are properly sized and installed in the correct orientation.

Low airflow across the evaporator coil:

1. Check the evaporator coil (located in the indoor unit) for any dirt, dust, or debris buildup.
2. Clean the coil using a coil cleaning solution and a soft brush, being careful not to bend the delicate fins.
3. Inspect the blower motor and fan for any issues that could be restricting airflow, such as a blocked intake or worn bearings.
4. Ensure the blower is operating correctly and providing adequate airflow across the evaporator coil.

Dirty condenser coil:

1. Turn off power to the outdoor condenser unit and remove the access panel.
2. Use a soft brush or coil cleaning solution to carefully clean the fins and coils of the condenser, taking care not to bend the delicate fins.
3. Vacuum or hose off any debris or dirt buildup on the coils.
4. Make sure the condenser area is clear of any obstructions or vegetation that could restrict airflow.

Thermostat issues:

1. Ensure the thermostat is set to the correct mode (cool) and the temperature setting is lower than the current room temperature.
2. Check the thermostat’s battery and replace it if necessary.
3. Verify the thermostat is accurately measuring the room temperature by comparing it to a separate thermometer.
4. If the thermostat appears to be malfunctioning, consider replacing it with a new, properly functioning unit.

Malfunctioning compressor:

1. Check the compressor’s electrical connections and ensure it is receiving the proper voltage.
2. Listen for any unusual noises or vibrations coming from the compressor, which could indicate a mechanical issue.
3. Use a multimeter to test the compressor’s starting and running capacitors, and replace them if they are faulty.
4. If the compressor appears to be the issue, it will likely need to be replaced by a licensed HVAC technician.

Low refrigerant charge:

1. Check the refrigerant level using a manifold gauge set. See if the pressure readings match the manufacturer’s specifications for your system.
2. Inspect the refrigerant lines for any visible leaks. Use a leak detection solution or an electronic leak detector to identify any leaks in the system.
3. If a leak is found, have the system professionally repaired and recharged with the proper refrigerant amount.
4. If no leaks are found, the system may simply need to be recharged by a licensed HVAC technician.

Refrigerant line restrictions:

1. Visually inspect the refrigerant lines for any kinks, crimps, or signs of damage.
2. Use a manifold gauge set to check the pressure drop across the refrigerant lines. A significant pressure difference may indicate a restriction.
3. If a restriction is found, have a licensed HVAC technician diagnose and repair the issue, as it may require specialized tools and techniques.

Oversized or undersized system:

1. Determine the cooling capacity of your current AC system (in BTUs) and compare it to the recommended size for your home’s square footage.
2. If the system is oversized, it may short-cycle and not effectively dehumidify the air. Consult an HVAC professional about installing a properly sized replacement unit.
3. If the system is undersized, it will struggle to maintain the desired temperature, especially during periods of high heat and humidity. Consider upgrading to a larger capacity system.

Aging or worn components:

1. Check the overall age and condition of the AC system, including the compressor, fan motors, capacitors, and other key components.
2. As the system ages, components can wear out or lose efficiency, leading to inadequate cooling.
3. If the system is nearing the end of its expected lifespan (typically 10-15 years), consider replacing the entire unit with a new, more energy-efficient system.

Additional information

Thank you for reading! We hope this guide helped you get your air conditioner working again.

If you have any other appliance repair needs or projects, don’t hesitate to explore our other troubleshooting guides and videos. 

If you need replacement parts or some extra help with ordering them, grab your model number and head over to AppliancePartsPros.com, where you can chat with a pro, read our DIY blog and more.

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Keep Reading:

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Choosing An Air Conditioner: How Many BTUs Do I Need?  

How to troubleshoot & fix common window air conditioner problems