AC Running But Not Cooling

Start with your thermostat and air filter

Before inspecting the equipment outside, check your indoor settings and airflow. A thermostat fan switch set to ON will keep the indoor blower running constantly, even when the system is not actively cooling. Make sure the thermostat is set to COOL and the temperature setting sits below the current room temperature.

Next, slide out your air filter to check for heavy dust. A clogged filter chokes the airflow through your system. When warm air cannot pass over the indoor evaporator coil, the coil can freeze into a solid block of ice. Once ice forms, air can still blow through the ducts, but the air will not cool the house.

• Check a 1 inch air filter every month, and plan to change it every 1 to 3 months.
• Make sure the airflow arrow on the filter frame points directly toward the furnace or air handler.
• Match the nominal size printed on the outer frame when replacing a dirty filter.

Inspect the outdoor unit and fan

Walk outside to your condenser while the system is calling for cooling. The outdoor fan needs to spin to push hot air out of your home. If the outdoor fan does not turn, heat builds up inside the unit until safety controls shut down the cooling cycle.

Listen closely to the outdoor cabinet. If the fan motor is silent, it might not be getting power. If the fan motor hums loudly but will not spin, the problem is often a failed capacitor rather than a broken motor. The capacitor gives the motor the electrical push it needs to start and run smoothly.

Turn off power at the breaker and the disconnect switch, test for power, and inspect the dual run capacitor. If the top of the can is swollen or bulging, it must be replaced. A dual run capacitor powers both the fan motor and the compressor using terminals labeled HERM, FAN, and C.

Check if the compressor is running

The compressor is the heart of your cooling system. It pumps refrigerant back and forth between the indoor and outdoor coils. If your outdoor fan blades are spinning, but the air blowing out the top of the unit feels cool instead of warm, the compressor may not be running at all.

A running compressor makes a steady, deep humming or vibrating sound. If you hear nothing from the compressor, or if it makes a buzzing click every few minutes, it may have trouble starting.

A failing capacitor or a worn contactor can prevent the compressor from turning on. The contactor uses a 24 V control signal from your thermostat to pull electrical contacts closed, sending 240 V to the compressor and fan. If the contact points are pitted or burned, high-voltage power cannot reach the compressor.

Clean a dirty outdoor coil

Your outdoor unit releases heat through a wrap-around coil made of thin metal fins. Over time, grass clippings, leaves, dirt, and cottonwood gather on the outside of these fins. This blanket of debris blocks air from flowing across the tubing.

When heat cannot escape into the outdoor air, the refrigerant stays hot. The compressor will overheat and cut out on its internal thermal overload switch to protect itself. Shut off power at the breaker and outdoor disconnect, test for power, and clear any dry leaves or weeds away from the cabinet. You can gently rinse dirt off the fins using low water pressure from a garden hose.

Check the condensate float switch

As your air conditioner cools the air, it removes humidity. That moisture collects in an indoor drain pan and flows down a drain line. If algae or dirt clogs that drain line, water backs up quickly.

Many modern systems use a float switch installed on the drain line or overflow pan. When water rises, the float switch trips and cuts the 24 V signal to the outdoor unit to prevent water leaks. The indoor fan will continue to blow air through your vents, but the outdoor unit will shut down completely. Inspect your drain pan to see if standing water is holding the float switch open.

Refrigerant problems require a certified technician

If your filters are clean, your coils are clear, and both the outdoor fan and compressor run, your system may have a refrigerant leak. An air conditioner does not consume refrigerant like a car consumes fuel. It operates as a sealed closed loop.

If refrigerant leaks out, the system cannot absorb heat from your indoor air. Signs of low refrigerant include weak cooling, hissing noises along the copper lines, or frost forming on the outdoor service valves and indoor coil.

Never attempt to measure, tap into, or refill refrigerant lines yourself. Refrigerant work requires specialized recovery tools, safety gauges, and an EPA Section 608 certified technician to locate and repair the leak according to federal law.