Indoor air is moving but not cool
Supply vents blow room-temperature or slightly warm air, and the outdoor unit is not running at all.
Start here: Start with thermostat mode, setpoint, filter condition, and outdoor power checks.
Air Conditioner Not Cooling
Indoor Unit Runs but Outdoor Unit Off
Direct answer: When the indoor unit runs but the outdoor unit stays off, the most common causes are a thermostat or power issue, a clogged filter causing a safety shutdown, or a condensate drain safety switch stopping cooling. If the outdoor unit has power but only hums, clicks, or tries to start, the problem is usually beyond basic DIY.
Most likely: Start with thermostat mode and setpoint, the air filter, the indoor drain safety switch, the main breaker, and the outdoor disconnect. Those checks solve a lot of these calls without opening equipment.
This symptom fools people because the house sounds like the AC is running. What is usually happening is the indoor blower is moving air, but the cooling side never comes on outside. Reality check: if the outdoor unit is completely silent, think power or a safety lockout before you think compressor failure. Common wrong move: resetting breakers over and over when the outdoor unit is trying to tell you there is a short or hard-start problem.
Don’t start with: Do not start by replacing the capacitor, contactor, or any hidden electrical part. Those are common failure points, but they are not safe guess-and-buy parts for most homeowners.
Outdoor unit totally silentCheck thermostat settings, breaker position, outdoor disconnect, and any full drain pan or float switch first.
Outdoor unit hums, clicks, or starts then stopsShut it down and arrange service instead of forcing more restart attempts.
Last reviewed: 2026-04-05
If the indoor fan runs but the outside AC unit never comes on, separate the silent-outdoor-unit cases from the humming-or-clicking cases first.
Outdoor unit is completely silent
No fan noise, no compressor hum, and no vibration at the outdoor cabinet.
Start here: Check the breaker, outdoor disconnect, and any condensate safety shutdown before anything else.
Outdoor unit clicks or hums but does not stay on
You hear a brief click, buzz, or hum outside, then nothing or a quick shutoff.
Start here: Turn the system off and stop at basic visual checks. That pattern usually needs a technician.
System stopped cooling after water around the indoor unit
The blower still runs, but you have a wet drain pan, water near the air handler, or a recent drain clog.
Start here: Look for a tripped condensate float switch or blocked drain line branch.
Most likely causes
1. Thermostat setting or call-for-cooling issue
The blower can run from fan settings or a partial thermostat problem while the outdoor unit never gets a proper cooling call.
Quick check: Set the thermostat to Cool, lower the setpoint at least 3 to 5 degrees below room temperature, and make sure Fan is on Auto, not On.
2. Power loss to the outdoor unit
A tripped breaker, pulled disconnect, or failed disconnect fuse can leave the indoor unit running while the condenser stays dead.
Quick check: Check for a tripped AC breaker and confirm the outdoor disconnect is fully seated and not obviously damaged.
3. Airflow or condensate safety shutdown
A badly clogged air filter or a backed-up drain can trip safeties that stop cooling while the blower still runs.
Quick check: Inspect the air filter and look for water in the drain pan, a wet floor, or a float switch near the indoor drain line.
4. Outdoor unit electrical or motor/compressor failure
If power is present but the unit only hums, clicks, or tries to start, the fault is often in a hidden electrical component or the compressor itself.
Quick check: Stand nearby during a cooling call. If you hear repeated clicking, buzzing, or a hot electrical smell, stop and call for service.
Step-by-step fix
Step 1: Confirm it is really calling for cooling
A lot of no-cool calls turn out to be fan-only operation, a schedule issue, or a thermostat that is not actually asking the outdoor unit to run.
- Set the thermostat to Cool.
- Lower the setpoint 3 to 5 degrees below the current room temperature.
- Set Fan to Auto instead of On.
- Wait up to 5 minutes in case the system is in a built-in delay.
- Listen for the outdoor unit after the delay ends.
Next move: If the outdoor unit starts and the air turns cold, the problem was a setting, schedule, or short delay rather than a failed AC component. If the indoor blower keeps running but the outdoor unit stays off, move to airflow and safety checks.
What to conclude: You have confirmed the symptom is real and not just the blower running by itself.
Stop if:
- The thermostat screen is blank and does not recover after basic battery or power checks.
- You smell burning plastic or hear loud buzzing from the thermostat or air handler.
Step 2: Check the filter and look for a drain safety shutdown
Restricted airflow and a backed-up condensate drain are common, safe-to-check reasons the cooling side stays off while the blower still runs.
- Turn the system off at the thermostat.
- Pull out the air filter and inspect it in good light.
- If the filter is packed with dust, replace it with the same size and airflow direction.
- Look around the indoor unit for water in the auxiliary pan, wet insulation, or a full condensate pan.
- If you can clearly see a condensate float switch sitting high because of water, address the drain problem before trying to run cooling again.
Next move: If a clean filter and cleared drain issue let the outdoor unit start normally, keep monitoring for the next full cooling cycle. If the outdoor unit is still off, check the power side next.
What to conclude: A dirty filter or tripped float switch can interrupt cooling without making the whole system look dead.
Stop if:
- There is standing water near electrical components.
- The indoor coil area is iced over or heavily frosted.
- You are not sure how to clear the drain safely without opening equipment.
Step 3: Check breaker and outdoor disconnect power basics
The indoor unit and outdoor unit often have separate power feeds. One can run while the other is completely dead.
- At the main panel, look for a tripped breaker labeled AC, condenser, or air conditioner.
- Reset a breaker only once by switching it fully off and then back on.
- Go to the outdoor unit and make sure the disconnect block is fully inserted if your setup has one.
- Look for obvious signs of trouble at the disconnect or whip, such as melted plastic, scorch marks, or loose hanging parts.
- Call for cooling again and listen for any change outside.
Next move: If the outdoor unit starts and runs normally after restoring power, keep an eye on it through the next day. A breaker that trips again is a service call, not a reset routine. If the breaker trips again, or the outdoor unit has power but only clicks or hums, stop DIY and arrange service.
Stop if:
- The breaker trips immediately or again after one reset.
- You see burned wiring, melted insulation, or arcing marks.
- The disconnect is damaged, loose, or hot to the touch.
Step 4: Separate a dead outdoor unit from a struggling outdoor unit
This is the point where the safe homeowner checks end and the repair path changes. A dead-silent unit is different from one that is trying and failing to start.
- Stand near the outdoor unit during a cooling call.
- Notice whether it is completely silent, gives one click, hums, or starts for a second and stops.
- Feel the cabinet only lightly with the back of your hand; do not remove panels. A very hot cabinet after failed starts is a bad sign.
- If the unit is silent and you already confirmed thermostat, filter, drain, breaker, and disconnect, stop at diagnosis.
- If the unit hums, clicks repeatedly, or smells hot, turn the thermostat off to prevent more strain.
Next move: If the unit starts and stays running smoothly, let it complete a cycle and verify cold air indoors. If it remains silent with power checks done, or it struggles to start, the next step is professional electrical and refrigerant-side diagnosis.
Stop if:
- You hear loud buzzing, hard clicking, or metal knocking.
- There is a burnt electrical smell.
- The fan blade is not turning but the unit is humming.
Step 5: Restore operation only if the simple cause was obvious, otherwise book service with a clear symptom report
Once the easy causes are ruled out, guessing at hidden AC parts gets expensive fast and can be dangerous.
- If you corrected a thermostat setting, replaced a clogged air filter, or resolved a clear drain safety issue, run the system for 15 to 20 minutes and check for steady cool air.
- If the outdoor unit still will not start, leave the thermostat off or set the system to Off to avoid repeated start attempts.
- Tell the technician exactly what you observed: silent outdoor unit, breaker status, any water at the indoor unit, and whether you heard clicking or humming.
- If the breaker tripped again or the unit hummed without starting, mention that first because it changes the service approach.
A good result: If cooling is back and stable, keep using the system and recheck the filter and drain over the next few days.
If not: If the outdoor unit stays off or acts like it wants to start but cannot, the repair is no longer a basic homeowner job.
What to conclude: You either solved a common setup or maintenance problem, or you narrowed it to a fault that needs safe testing under load.
Stop if:
- Anyone suggests repeated breaker resets as the fix.
- You would need to open the outdoor electrical compartment to continue.
- The system starts tripping, smoking, or making harsher noises than before.
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FAQ
Why does my indoor AC fan run when the outside unit is off?
Because the blower and the outdoor condenser are separate parts of the system. The thermostat can run the indoor fan by itself, or the outdoor side can be shut down by a power problem, drain safety switch, or equipment fault while the blower still moves air.
Can a dirty filter keep the outdoor unit from turning on?
It can. A severely clogged air conditioner filter can cause airflow problems and icing, and some systems end up in a safety shutdown or no-cool condition while the blower still runs. It is one of the first things worth checking because it is safe and common.
Should I reset the breaker if the outdoor unit will not start?
You can reset it once if it is tripped. If it trips again, stop there. A repeat trip usually means a real electrical or compressor-side problem, and repeated resets can make the damage worse.
What if the outdoor unit just hums or clicks?
That is different from a unit that is completely dead. Humming or clicking means it is trying to start and cannot. That often points to a failing start circuit, fan motor issue, or compressor problem, and it is not a good DIY path for most homeowners.
Could a clogged drain stop my AC from cooling?
Yes. Many systems use a condensate float switch to shut off cooling when the drain backs up, so water does not overflow into the house. If you have water near the indoor unit and the blower still runs, that is a strong clue.
Is this the same as an AC blowing warm air?
Sometimes, but not always. If the outdoor unit is completely off while the indoor blower runs, this page fits better. If the outdoor unit does run and you still get warm air, the problem is different and usually needs a separate diagnosis.