An air conditioner that is running but not cooling is a different problem from one that will not turn on at all. The outdoor unit is humming. The indoor blower is moving air. Everything sounds normal. But the air coming out of the registers is room temperature, or only slightly cooler than room temperature, and the house is not cooling down. The equipment is consuming electricity without producing comfort. The problem is in the refrigeration cycle. The compressor may be running, but heat is not being transferred from the indoor air to the outdoor air. The cause is almost always one of four things: the outdoor condenser coil is too dirty to reject heat, the indoor evaporator coil is frozen, the system is low on refrigerant, or the airflow through the system is so restricted that no meaningful cooling can occur.
The first diagnostic step is to check the temperature of the air coming out of the registers. Hold a thermometer in the supply air stream for a minute. The air should be 15 to 20 degrees cooler than the air entering the return grille. If the temperature drop is less than 15 degrees, the system is not cooling effectively. If the temperature drop is zero or close to it, the compressor may not be running despite the sound of the outdoor unit. The outdoor unit has two motors: the compressor and the condenser fan. The fan may be running while the compressor is not. The fan is loud. The compressor is a low hum. It is possible to hear the fan and assume the compressor is running when it is not. The temperature drop tells you whether cooling is happening at all.
EPA WaterSense encourages regular maintenance of home systems to ensure efficient operation. An AC that runs without cooling is the most wasteful condition an HVAC system can experience, consuming electricity continuously without providing any benefit.
Cause #1: Dirty Condenser Coil — Can’t Reject Heat
The most common cause of an AC running but not cooling adequately is a dirty outdoor condenser coil. The condenser coil rejects the heat that the evaporator coil absorbed from the indoor air. When the condenser coil is packed with dirt, grass clippings, cottonwood fluff, leaves, or pet hair, it cannot reject heat efficiently. The refrigerant stays warm. Because the refrigerant is warm, the evaporator coil cannot absorb much heat from the indoor air. The air coming out of the registers is cool but not cold. The AC runs continuously. The house never reaches the set point. The longer it runs, the hotter the condenser gets, and the less cooling it produces.
Clean the condenser coil. Turn off power at the disconnect switch next to the outdoor unit. Remove any debris from around the unit. If possible, remove the top and the fan assembly to access the coil from the inside. Spray the coil from the inside out with a garden hose and a nozzle set to moderate pressure. Do not use a pressure washer. The high-pressure spray bends the aluminum fins flat and permanently reduces the coil’s ability to transfer heat. Spray until the water runs clear. If the coil is greasy, use a commercial condenser coil cleaner. Let it foam and rinse according to the product instructions. Reassemble the unit and restore power. The cooling performance should improve immediately if the dirty condenser was the cause.
Cause #2: Frozen Evaporator Coil — Ice Blocking Airflow
A frozen evaporator coil blocks airflow. The ice acts as an insulator on the coil. The refrigerant cannot absorb heat through the ice. The air passes around the ice rather than through the coil. The result is air that is barely cooler than room temperature. The AC runs continuously. The ice gets thicker. The cooling gets worse.
The coil freezes because the airflow across it is restricted or the refrigerant pressure is too low. A dirty air filter is the most common cause of restricted airflow. Check the filter. If it is dirty, replace it. Turn the AC off and set the fan to “On” at the thermostat to blow warm air across the frozen coil. The thawing takes one to four hours. Place towels under the air handler to catch meltwater. Do not chip at the ice. Chipping can puncture the coil and cause a refrigerant leak. Once the ice is completely melted and the filter is replaced, restart the system. If the coil freezes again, the problem is low refrigerant or a more severe airflow restriction, such as a dirty blower or undersized ductwork.
Cause #3: Low Refrigerant — The Leak You Cannot See
Low refrigerant produces the same symptoms as a dirty condenser coil: reduced cooling, long run times, the house not reaching the set point. The compressor runs. The fans run. The system sounds normal. But the refrigerant charge is low, either because it was undercharged at installation or because a leak has developed over time. An AC does not consume refrigerant. If the level is low, refrigerant has escaped.
The refrigerant pressure in the evaporator coil is too low to absorb heat effectively. The suction line, the larger of the two copper lines at the outdoor unit, should be cold and sweating on a hot day. If the suction line is cool but not cold, or is at ambient temperature, the system is low on refrigerant. The evaporator coil may be partially frozen, with ice forming first at the point where the refrigerant enters the coil and spreading across the coil as the charge gets lower.
Diagnosing low refrigerant requires a technician with pressure gauges. The technician will measure the pressures, locate the leak, repair it, evacuate the system, and recharge it to the manufacturer’s specification. The repair costs $300 to $1,500. A system that uses R-22 is more expensive to recharge. A system more than 10 years old with a major refrigerant leak may be a candidate for replacement.
Cause #4: Restricted Airflow — Filter, Returns, and Ducts
Even with a clean condenser and a full refrigerant charge, the AC cannot cool the house if the airflow through the system is severely restricted. The airflow carries heat from the house to the evaporator coil. If the airflow is restricted, less heat reaches the coil. The system cannot remove what it does not receive. The symptoms are the same as a dirty condenser: the system runs, the air is cool but not cold, and the house does not reach the set point.
The restrictions can be on the return side or the supply side. A dirty filter is the most common return-side restriction. Closed or blocked return grilles are the second. Furniture pushed against a return grille, a rug hung over it, or a return that has been blocked to redirect airflow all starve the blower of air. On the supply side, closed supply vents, furniture blocking registers, and undersized or crushed ductwork restrict the delivery of cooled air to the rooms. Walk through the house and check every return and every supply register. Open them all. Clear any obstructions. The fix is as simple as moving a piece of furniture or opening a vent that has been closed for years.
Frequently Asked Questions
How do I check if the AC is cooling by touching the pipes?
The larger copper pipe at the outdoor unit, the suction line with the insulation on it, should be cold to the touch and sweating on a hot humid day. The smaller pipe, the liquid line, should be warm. If the suction line is not cold, the system is not transferring heat effectively. If the suction line is frozen, the coil is frozen. If both pipes are the same temperature, the compressor may not be running despite the fan running.
Can a compressor run but not pump refrigerant?
Yes. A compressor has internal valves that open and close as the piston or scroll moves. If the valves fail, the compressor runs but does not pump refrigerant. The motor runs. The sounds are normal. But no pressure differential is created. No cooling happens. This is an internal compressor failure and requires replacing the compressor or the entire condensing unit. It is less common than a dirty coil or low refrigerant but should be considered if those causes are ruled out.
Why does my AC cool at night but not during the day?
The outdoor temperature is lower at night. The condenser can reject heat more easily into cooler air. A system that is marginal, dirty condenser, slightly low refrigerant, or undersized, may cool adequately at night when the load is lower and fail to cool during the heat of the day when the load is highest. The underlying problem is still present. The nighttime cooling masks the symptoms. The daytime failure reveals them. Clean the condenser coil first. If the problem persists, have the refrigerant charge checked.
The Bottom Line
An AC that runs but does not cool has a problem in the refrigeration cycle. The condenser coil may be too dirty to reject heat. The evaporator coil may be frozen from a dirty filter or low refrigerant. The refrigerant may be low from a leak. The airflow through the system may be restricted by closed vents or blocked returns. Clean the outdoor coil first. Replace the filter second. Check for ice on the indoor coil third. Measure the temperature drop at the registers fourth. If the temperature drop is less than 15 degrees, and cleaning and filter replacement do not improve it, call a technician for a refrigerant diagnosis. The AC that runs and runs without cooling is consuming electricity and wearing out its compressor for nothing. Fix it and the house will be cool again within hours.
Last modified: July 7, 2026
