AC fan not running? Find which fan stopped and why

Which AC fan stopped, the indoor blower or the outdoor fan?

Start here, because the answer decides everything that follows. A central air conditioner has two separate fans driven by two separate motors, and they fail for different reasons. The indoor blower lives in the furnace or air handler and pushes cold air through the ducts to your registers. The outdoor fan sits on top of the outside unit and blows heat off the coil. They run off different parts, so a dead one on one side tells you almost nothing about the other.

The fastest way to tell them apart is the thermostat fan switch. Set it from AUTO to ON. That should force the indoor blower to run continuously, whether or not the system is cooling. Then walk to a vent. If you feel a steady stream of air, the indoor blower works. If you feel nothing, the indoor blower is your problem. Next, go outside and look at the fan blade on top of the unit while the system is calling for cooling. If it spins, the outdoor fan is fine. If the unit hums or buzzes but the blade sits still, the outdoor fan is your problem. Those two checks sort almost every case into one of the buckets below.

One result rules out fans entirely: if both fans run but the house still will not cool, this is not a fan problem at all. That points to refrigerant or airflow, and our guide on an AC that runs but will not cool covers it. And if nothing comes on anywhere, no indoor fan, no outdoor unit, no thermostat response, that is a power or control problem better handled by our guide on an AC that will not turn on.

Why won't the indoor blower run when no air comes out of the vents?

If the thermostat is calling for cool and the blower will not move air, work the free checks first, because several of them are common and cost nothing. Find the breaker for the furnace or air handler and reset it once; if it trips again immediately, stop, because that signals a short or a seized motor, not a nuisance trip. Make sure the furnace door or blower access panel is fully closed and latched, because a safety switch behind it cuts blower power whenever the panel is loose. Check the air filter, since a badly clogged one chokes airflow enough to overheat the blower and trip a safety that shuts it down. And check the condensate drain, because when the drain line clogs and water backs up, a float switch shuts the indoor unit off to stop a flood.

Frozen coil is the other free-to-spot cause, and there is a quick test to separate it from a dead blower. Set the fan to ON and feel a vent. If you feel no air at all, the blower itself is dead. If you feel air but it is weak, the blower is running and something downstream is blocking it, often ice. Look at the indoor coil and the refrigerant line for frost. A coil iced over blocks the air, and running the system that way risks the compressor. Switch cooling off, set the fan to ON to thaw the ice, which can take a few hours, and change the filter, since restricted airflow is usually what freezes a coil in the first place. The freeze is often a symptom of a weak blower or a dirty filter rather than a fault on its own, so fixing the airflow fixes the ice.

If the free checks come up empty, you are into parts a technician replaces. The blower motor itself can fail from worn bearings or a burnt winding. On older systems the motor relies on a run capacitor for the push to start spinning, and a dead capacitor leaves the motor humming but stuck. Newer variable-speed blowers, called ECM motors, have no capacitor but carry an electronic module that can fail on its own, sometimes replaceable by itself for less than the whole motor. A failed control board or fan relay, or a blown low-voltage fuse on that board, can also leave a perfectly good blower with no signal to run. One useful clue: if the blower runs when you set the fan to ON but not during an automatic cooling call, the motor is fine and the fault is in the control path, the board or the relay.

There is also a weaker version of this that fools people. If the blower spins but the air at the vents is feeble, the motor is not dead, something is choking the airflow. The usual culprits are a clogged filter, a blocked or closed return grille, or a duct that has come apart in the attic. A failing motor that is on its way out can also start slow or drop out partway through a cycle. Weak air is a different problem from no air, and it usually traces to the filter or the ducts before the motor.

Could one problem stop both fans at once?

Yes, and it is worth knowing which faults are shared, because they change where a tech looks. Both fans draw their control signal from the same low-voltage circuit, a 24-volt loop protected by a small three- to five-amp fuse on the control board and fed by a transformer. If that fuse blows or the transformer fails, the controls go dead and neither fan gets the message to run, even though both motors are fine. A blown control fuse that pops again the moment it is replaced means there is a pinched or chewed thermostat wire shorting out somewhere, and that wire has to be found before the system is safe to power back up.

On the outdoor side, the contactor is the shared switch that feeds line power to both the outdoor fan and the compressor. When its contacts burn or stick, the whole outdoor unit can go quiet while the indoor blower runs on fine. So the pattern tells the story: both sides dead usually points to the low-voltage circuit or a tripped main, the outdoor unit alone dead points to the contactor or its power, and the indoor blower alone dead points to the blower motor, its board, or a tripped safety. A single fan stuck while everything else works is almost always that fan's own motor or capacitor.

Why is the outdoor unit humming but the fan won't spin?

This is the most common fan failure of all, and it has a classic signature: the outdoor unit hums or buzzes, but the fan blade on top does not turn. Nine times out of ten that is a failed capacitor. The capacitor is the part that delivers the jolt the fan motor needs to start spinning, and when it dies, the motor tries, cannot get over the hump, and just sits there humming. Some people confirm it by nudging the blade with a stick and watching it spin up, which points straight at the capacitor, but that is a confirmation, not a fix, and you should not leave it running afterward.

Here is the part that matters most: a capacitor is strictly a technician's job, not a do-it-yourself one. It stores a high-voltage charge and holds onto it even after you cut the power, enough to deliver a serious or even fatal shock. Killing the breaker does not make it safe. Do not open the electrical panel on the outdoor unit and do not touch the capacitor. A tech discharges it safely with the right tool before going near it. Beyond the capacitor, an outdoor fan can also be stopped by a failed fan motor, a worn contactor not passing power to the unit, or debris physically jamming the blade, which you can clear only after the power is off.

There is real urgency to shutting it down. The outdoor fan exists to cool the compressor, the single most expensive part in the system. When that fan stops while the compressor keeps running, the compressor overheats fast, and an overheated compressor can be destroyed outright. So a humming unit with a still fan is a shut-it-off-and-call situation, not a run-it-and-watch one. Letting an eighty-dollar capacitor failure cook the compressor can turn it into a repair that costs thousands or a full system replacement.

When should I shut the AC off right away?

Three signs mean cut the power now, before you do anything else. The first is any motor that hums or buzzes but does not spin, indoors or out. A stalled motor draws a heavy starting current the whole time it sits there, which overheats it within minutes and, on the outdoor side, overheats the compressor with it. Shut it off and leave it off until a tech looks at it. The second is a burning or scorched smell, which means wiring insulation is overheating and is a genuine fire risk; kill power at the breaker and call. The third is a breaker that trips again the instant you reset it, which means a short somewhere downstream, not a fluke.

None of these are wait-and-see. Each one gets worse the longer the system runs, and each one moves you from a cheap part toward an expensive one. Cutting power costs you nothing and protects the compressor, which is the part you cannot afford to lose.

What do these AC fan repairs cost?

Once the free checks are done, a dead fan is a tech repair, and the part involved sets the price. These are installed costs, parts and labor together, so you know whether you are looking at a small fix or a big one before the truck arrives. Regional labor rates and warranty status move them around.

Repair	Installed cost	What it fixes
Run or dual capacitor	$80 to $400	The starting jolt for the outdoor fan and compressor. The most common, cheapest fix.
Contactor	$150 to $450	The switch that feeds power to the outdoor unit on a cooling call.
Condenser fan motor	$200 to $700	Spins the outdoor fan. Worn bearings or a burnt winding stop it.
Blower motor, older PSC type	$300 to $800	The indoor fan on a standard single-speed system.
Blower motor, variable-speed ECM	$600 to $1,500	The indoor fan on a high-efficiency system. The module alone is sometimes cheaper.
Control board or fan relay	$150 to $700	The board that tells a fan to run. A failed relay leaves a good motor with no signal.
Low-voltage fuse or transformer	$100 to $400	Powers the 24-volt control circuit. A blown fuse can leave the controls dead.

The cheapest of these, the capacitor, is also the most common, which is the good news in most fan failures. The thing that turns a cheap repair into an expensive one is running the unit while it hums, so the single best money-saving move is shutting it off the moment a motor stalls. If a tech finds the compressor itself is damaged on an older unit, price a full replacement before you commit, because our replace-versus-repair calculator often shows a big compressor bill on aging equipment is money better put toward a new system.