Troubleshooting a central air conditioner that runs but does not cool

As the hot summer weather arrives, few things are more frustrating than an air conditioning system that seems to be running, but just can’t seem to keep your home cool and comfortable. If your central AC unit is blowing air but not providing the cooling relief you expect, we’re here to help you diagnose and fix the cooling issues in your home’s air conditioning system.

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
• Screwdrivers
• Coil cleaner
• Vacuum cleaner or compressed air

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.

Troubleshooting steps

Check for airflow blockages:

1. Inspect the indoor and outdoor unit for any obstructions or restrictions to airflow.
2. Clear away any debris, dirt, or objects blocking the air intake or exhaust grilles.
3. Ensure there is adequate clearance around the outdoor unit for proper airflow.
4. Check that the ductwork is not blocked, crushed, or disconnected anywhere in the home.
5. A clogged or dirty air filter can severely restrict airflow through the system. Replace the filter with a new, clean one of the proper size.
6. Clean or replace the filter on a regular maintenance schedule.

Inspect for evaporator coil icing:

1. Visually examine the indoor evaporator coil for any ice buildup.
2. Ice on the coil indicates issues like low refrigerant charge, airflow problems, or a faulty defrost control.
3. Turn off the system and allow the ice to fully melt before restarting.
4. Over time, the indoor evaporator coil can become coated in dirt, dust, and grime.This buildup insulates the coil and impedes heat absorption.
5. Carefully clean the coil using a coil cleaner and a soft brush or compressed air.

Verify thermostat or temperature control:

1. Ensure the thermostat is set to a temperature below the current room temperature.
2. Check that the thermostat and temperature control switch are operating correctly.
3. Replace the thermostat if it appears faulty or is not accurately sensing the temperature.

Check the fan switch and motor:

1. Locate the fan control switch, which is often part of the thermostat or in a separate control box.
2. Set the fan switch to the “On” position and verify that the indoor blower fan is running.
3. If the fan is not running with the switch in the “On” position, the switch may be faulty and need replacement.
4. Locate the fan motor capacitor, which is often mounted near the fan motor.
5. Use a multimeter to test the capacitor. Set the multimeter to the capacitance (μF) setting.
6. Discharge the capacitor by shorting the terminals with an insulated screwdriver.
7. Measure the capacitance and compare it to the rated value on the capacitor label.
8. If the capacitance is significantly lower than the rated value, the capacitor needs replacement.
9. Inspect the fan motor terminals and wiring connections for any signs of damage, corrosion, or loose connections.
10. Use a multimeter to check for continuity between the motor terminals and the power source.
11. Repair or replace any damaged or malfunctioning terminals or connections.
12. If the fan is not running at all, the fan motor itself may be faulty.
13. Carefully spin the fan blades by hand to check for any binding or seizing of the motor bearings.
14. If the fan motor feels stuck or the bearings are bound, the motor will likely need replacement.

Test the compressor capacitor and relay:

1. Locate the start capacitor, which is often mounted near the compressor.
2. Use a multimeter to test the capacitor. Set the multimeter to the capacitance (µF) setting.
3. Discharge the capacitor by shorting the terminals with an insulated screwdriver.
4. Measure the capacitance and compare it to the rating on the capacitor label.
5. If the capacitance is significantly lower than the rated value, the capacitor needs replacement.
6. Also visually inspect the capacitor for any signs of bulging, leaking, or other damage.
7. Locate the compressor relay, which is often mounted near the contactor or in the control box.
8. Set the multimeter to the resistance (Ω) setting.
9. Identify the common (C), normally open (NO), and normally closed (NC) terminals on the relay.
10. Measure the resistance between the C and NO terminals – it should be very low (near 0Ω) when the relay is energized.
11. Measure the resistance between the C and NC terminals – it should be very high (near ∞Ω) when the relay is energized.
12. If the relay is not switching correctly, it needs to be replaced.

Additional information

Thank you for reading! We hope this guide helped you get your central 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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