AC Not Blowing Cold Air: Causes and Fixes to Check First

An AC not cooling, or an AC not blowing cool air, usually comes down to one of the causes below, and the checks are the same for a central AC not blowing cold air or an AC not cooling house. How to fix AC not blowing cold air depends on which part has stopped doing its job, so the sections below go in the order a technician would check them.

Often part of the system still works: AC fan running but not cooling, AC is running but not blowing cold air, AC on but not cooling, or AC blowing but not cold. That detail is useful, because something running means the power and the thermostat are fine and the fault is in the cooling itself. Timing is the other clue. An AC unit not cooling from one hour to the next has usually lost a part, and an AC not getting cold a little more each week is losing airflow or refrigerant. So before you check anything, settle two things: what is still running, and whether it stopped cooling suddenly or slowly.

AC Running But Not Cooling

Work through the free checks before anyone gets paid. In order:

• Thermostat mode and fan setting. Set to cool, and the fan on AUTO rather than ON. This one catches people constantly: with the fan set to ON it runs continuously and pushes uncooled air through the vents between compressor cycles, which feels exactly like a system that has stopped cooling.
• The air filter. The most common single cause, and the cheapest.
• Breakers. Indoor and outdoor units are usually on separate circuits, so the inside can run while the outside is dead.
• Is the outdoor unit actually running? Fan turning, compressor audible. A humming unit with a still fan is often a failed capacitor.
• Is the indoor coil iced? If yes, stop and go to the next section.

If all of that is clear, the remaining causes cost money: a capacitor, a contactor, a refrigerant leak, or a compressor. A capacitor is commonly a $150 to $250 job, and the AC repair cost calculator prices each of the others.

And the one to be firm about: refrigerant is not consumed. It circulates in a sealed loop and does not deplete with use. As one technician put it, it is neither created nor destroyed inside the system. So a unit that is low on refrigerant has a leak, period. Topping it up without finding that leak is refilling a bucket with a hole in it, and a contractor who offers to do that every spring is selling you the same repair annually rather than fixing anything.

AC Not Blowing Cold Air: What Each Symptom Points To

An AC not blowing cold air is the same complaint as an AC not cooling, and the checks above cover both. The same checks apply to an AC unit not blowing cold air or a central air conditioner not blowing cold air, and to a home AC not cooling the house or an air conditioner running but not cooling. What narrows it down is the exact way it fails. Match what you see and hear to the row below before you call anyone, because half of these cost nothing to fix.

Tip: if the air conditioner is not blowing cold air and the outdoor unit is running normally, stop at the filter and the coil before you suspect refrigerant. Airflow problems are far more common than leaks, and they cost a few dollars instead of a few hundred.

AC Freezing Up

Ice on the coil has two causes and they present identically to the owner, which is why so much money gets spent on the wrong one. It does not matter where you first see it: an AC unit freezing up at the indoor coil, an AC line frozen at the outdoor unit and a frozen air conditioner in a window are all this section.

Not enough air over the coil. A dirty filter, closed or furniture-blocked vents, a starved return, a failing blower, or ductwork too small for the system. With too little warm air passing over it, the coil temperature falls below freezing and condensate turns to ice instead of draining. If undersized ducts are the suspect, our duct size calculator shows what the system actually needs.

Not enough refrigerant. Lower pressure means a lower coil temperature, with the same result. And per the section above, that means a leak.

How to tell them apart without paying anyone: switch cooling off and run the fan alone until the ice is completely gone, which takes a few hours rather than minutes. Fit a clean filter, open every vent, and run it again. If it stays clear, it was airflow and you have fixed it. If it ices up again with a clean filter and open vents, it is refrigerant, and that needs a professional with recovery equipment.

Do not run cooling while the coil is iced. A flooded coil sends liquid refrigerant back to a compressor built to handle gas, and that is how a dirty filter becomes a compressor replacement.

Quick Fix for an AC Freezing Up, and What a Frozen Line Means

The quick fix for an AC freezing up is the same on every system: set the thermostat to off, set the fan to ON, and let room air melt the ice. Do not chip at it and do not use a heat gun, because the fins and the tubing are thin. A frozen AC unit needs two to four hours to clear, longer if the coil is a solid block.

Where you see the ice tells you little, because it all comes from one place. A frozen AC line outside, the thick insulated pipe at the condenser, means the evaporator coil indoors froze first and the ice has traveled down the line. So an AC unit frozen outside and a frozen evaporator coil are the same fault seen from two ends. An AC that keeps freezing up after a clean filter and open vents has a refrigerant leak, and a leak repair with a recharge runs $200 to $1,500.

Frozen AC Unit: Coil, Line or Outside, and How to Unfreeze It

A frozen AC unit shows its ice in three places, and people name the fault after the place they saw it. An AC evaporator coil frozen behind the indoor panel is where it always begins. From there the ice spreads along the large copper pipe, which is why AC line freezing or an AC pipe frozen at the outdoor unit comes later, and an AC frozen outside at the service valves comes last. Call it AC froze up, AC unit frozen up or air conditioner freezing up: the order never changes. By the time the outdoor end is white, the indoor coil has been a block of ice for hours.

The answer to why is my AC freezing up starts with getting the ice off. How to unfreeze AC unit ice is the same wherever it shows: cooling off, fan on, and patience. Two things bring it back. One is running the system on a cool night, because with mild air outside the coil runs colder than it was built for, so an AC that freezes only at night may have nothing wrong with it. The other is the usual pair, airflow and refrigerant. Tip: if the AC keeps freezing up in the heat of the day with a clean filter, stop thawing it and have the leak found.

Low Refrigerant Symptoms: How an AC Freon Leak Shows Itself

The signs of a low charge build slowly, and that is what separates them from an electrical fault, which arrives all at once. Look for these together.

• Cool, never cold. The air at the vent loses a little each week.
• Longer runs. The system takes more hours to do the same job, and the bill shows it.
• Ice with a clean filter. Frost on the coil or the large copper pipe when the airflow is fine.
• A hiss or a bubbling sound. At the indoor coil or along the pipes, while it runs.
• Oily residue at a joint. Refrigerant carries oil with it, and the oil stays behind where the gas escaped.

The same list is what AC low on freon symptoms look like. An AC freon leak is the only way a system gets there, because nothing in normal running uses the refrigerant up, and finding the leak and recharging runs $200 to $1,500. Tip: one item from this list is a dirty filter until proven otherwise. Several together point to a leak.

AC Blowing Warm or Hot Air

An AC blowing warm air and an AC not blowing cold air look like one problem, but warm air narrows it down. Room-temperature air means the cooling side is not running. Air that is clearly hot means something is adding heat. So an AC blowing hot air and an AC blowing room temperature air are two different faults, and the list below is in that order.

• Thermostat set to heat, or fan set to ON. Check the mode before anything else.
• The outdoor unit is off. The blower alone just moves house air around. Follow the checks for an outside AC unit not turning on.
• A new thermostat on a heat pump. Heat pumps use a reversing valve, and the thermostat has a setting, usually labeled O/B, that tells it which way the valve works. Set wrong, the system heats when you ask for cooling. Owners who swap a thermostat and get hot air from the vents have almost always hit this.
• Ducts in a hot attic. The first minute of air is warm because the duct itself is hot. If it turns cold after that, nothing is wrong.
• A leaking return duct. A return that has pulled apart in the attic draws in 130 degree air and mixes it with the cooled air.
• Low refrigerant or a dirty outdoor coil. Both leave the air only slightly cool. The symptom table above separates them.

Tip: if your AC is blowing hot air right after a thermostat change, the thermostat is the cause until proven otherwise. Check its heat pump setting and its wiring before paying for a service call.

Window AC, Mini Split and Heat Pump Not Cooling

The checks above are written for a central system. The other types fail in the same ways, with a few differences worth knowing. A window AC not blowing cold air, a mini split that has gone warm and a heat pump not cooling each have one check the others do not.

Window AC not cooling. Clean the filter behind the front grille, then look at the back of the unit from outside. The rear coil clogs with lint and cottonwood, and a window air conditioner not blowing cold air but running is very often just that. Also check the mode: fan-only and energy saver settings both blow uncooled air between cycles.

Mini split not cooling. Wash the mesh filters under the front panel, which most owners never do. Check the remote is in cool mode and not dry or fan mode. A head that blinks a code is reporting a fault the manual will name. Beyond that, a mini split that has lost its charge usually has a leak at a flare joint, which is an installation fault.

Heat pump not blowing cold air. Everything on this page applies, plus one extra part: the reversing valve. A heat pump stuck in heating mode blows warm air in summer with every other part working. The thermostat setting described above is the first check.

AC Not Cooling Enough, or Not Cooling Below 75 or 80

An AC that cools but stalls a few degrees short is a different complaint from one that does not cool at all. If it happens only on the hottest afternoons, it may be working exactly as designed, and the 20 degree rule for air conditioners explains what a system can and cannot do. If it used to hold the setpoint and now cannot, something has changed: a dirty outdoor coil, a filter overdue, a duct that has come apart, or refrigerant slowly leaking out.

AC Not Keeping Up or Running Constantly on Hot Days

An AC not keeping up in the late afternoon is the most common complaint of a heat wave, and often there is no fault to find. A system is sized for a typical hot day in its climate, so on the few days hotter than that it runs without stopping and still drifts a few degrees above the setting. An AC running constantly through those hours is doing all it can, and an AC not cold enough on those afternoons, or an AC not cooling below 75, says as much about the weather as about the machine.

Three signs say it is a fault. It runs through the night as well, after the sun is off the house. It kept up last summer in the same heat. Or the air at the vent is only slightly cooler than the room. Dirty condenser coil symptoms fit this picture exactly: fine in the morning, fading by mid-afternoon, and better again once the coil has been hosed clean. Tip: if an AC runs constantly only in the hottest hours, look at shade, attic insulation and the outdoor coil. If it never stops at all, look for a fault.

AC Not Blowing Cold Air After Power Outage

Power coming back is harder on an air conditioner than power going off. With AC not blowing cold air after power outage as the complaint, work through these in order.

1. Wait. The compressor is held off for several minutes after any interruption, so the fan may run alone at first.
2. Check the outdoor breaker. Everything in the house restarts at the same moment, and the AC breaker is the one that most often trips in that rush.
3. Check the thermostat. Some lose their settings and come back in off or heat mode, or with the schedule wiped.
4. Then suspect the capacitor. A surge finishes off a part that was already weak, and a new capacitor or contactor costs $150 to $450 fitted.

Tip: a system that cooled before the outage, with air still coming from the vents after it, has nearly always lost only its outdoor power. Start at the breaker, not at the thermostat.

AC Not Cooling Upstairs or in One Room

When the rest of the house is comfortable the machine is working, and the problem is in how the air is delivered. With an AC not cooling upstairs, heat collects on the top floor and the longest duct runs end there, so the rooms that need the most air get the least. One warm room points at that room alone: a damper shut by mistake, flex duct flattened under stored boxes, or a run that has come loose from its register. The duct size calculator shows what a run of that length should carry. Tip: one warm room is a duct, a whole warm floor is balance, and neither is fixed by a bigger air conditioner.

AC Not Cooling: When to Stop and Call

The free list is short: thermostat mode and fan setting, filter, breakers, a clean outdoor coil and a thawed indoor coil. Past that point every remaining cause needs gauges or a meter. In rising order of cost they are a capacitor or contactor at $150 to $450, a refrigerant leak found and recharged, an evaporator coil at $1,000 to $2,500 and a compressor at $1,000 to $2,500. On a system past the middle of its life, the last two are the point to open the central air installation cost calculator and compare. Tip: do the free list once, properly. If the air is still not cold, a second pass will not change it.