AC running but not cooling? 14 causes, in order of likelihood
An AC that runs without cooling has one of 14 causes, listed below from most to least likely — 5 are free checks, 4 are cheap fixes, and 5 need a licensed technician. Work them in order; each shows the fix and what it costs.
If it's dangerously hot inside, call (888) 810-2291 first and run these checks while you wait for a licensed local contractor.
First: should you turn the AC off?
Yes — if you see ice anywhere on the unit or lines, or hear grinding. Running it then strains the compressor and worsens the icing, and grinding is mechanical damage compounding with every minute. Otherwise, leave it running and check the causes in order — most are safe to diagnose with the system on.
The 5 free checks first (causes 1–5)
Five causes cost nothing to rule out. Do them in this order before spending a dollar or booking a visit.
1.Thermostat set wrong$0
Confirm it's on COOL, the setpoint is at least 3°F below room temperature, and the fan is on AUTO, not ON. Fan-ON blows uncooled air through the vents between cooling cycles — the classic "running but warm" illusion. Batteries die silently and freeze the whole system. Set it right, then give it 15 minutes before moving to the next check.
2.Dirty air filter$5–$40
Pull the filter and hold it up to a light — no light through means it's replaced. A clogged filter collapses airflow across the coil, the coil ices, and the vents blow warm. A 1-inch filter needs swapping monthly to quarterly. Wait 30–60 minutes after a fresh filter before you judge whether it fixed things.
3.Blocked vents and returns inside$0
Walk every room: furniture on supply vents, curtains over returns, and closed vents in unused rooms all starve airflow. Closing vents does not save energy — it unbalances the system and can ice the coil. If one room is warm while the rest are fine, start here.
4.Tripped breaker or outdoor disconnect$0
The indoor blower and the outdoor unit run on separate circuits. Indoor fan running but the outdoor unit dead silent points to a tripped outdoor breaker. Reset it once. If it re-trips, stop — that is an electrical fault and a fire risk, not a reset problem. Check the outdoor disconnect box too; the fuse block can be pulled loose after storms or mowing.
5.Heat pump stuck in heating mode$600–$1,200
If your system is a heat pump and blows warm air on COOL, the reversing valve or thermostat wiring may be stuck in heat mode. Check the thermostat mode; the outdoor unit blowing hot air upward in cooling is normal, but warm air from the indoor vents is not. A confirmed reversing-valve fault is a technician repair — see our heat pump repair page for the full problem list.
Free checks done, still warm?
Enter your ZIP — we'll connect you to a licensed local contractor for the diagnosis. The call is free, 24/7.
In a hurry? Call (888) 810-2291 now.
Licensed contractors serve . One call routes you to one for .
☏ Call (888) 810-2291Availability is subject to provider participation, location, technician availability, and demand.
The 4 cheap-to-moderate fixes (causes 6–9)
These cost money, but small money — roughly $100–$700 — and two have DIY halves you can do safely.
6.Frozen evaporator coil$150–$1,600
Ice on the refrigerant lines or the indoor unit means stop cooling and run the fan only for 2–3 hours to thaw it before a technician arrives — a thawed coil can be diagnosed on the visit, which saves you a return-trip fee. The root cause is airflow (filter/vents) or refrigerant. If the ice comes back after a full thaw, stop and call.
7.Dirty condenser coil (outdoor unit)$150–$400
An outdoor coil coated in cottonwood, grass, and dust can't shed heat, so the AC runs all day and the house never cools. DIY half: power the unit off, then gently rinse the coil top-down with a hose. Bent fins or dirt packed inside the coil is a professional cleaning. It's worst after cottonwood weeks and mowing season.
8.Clogged condensate drain line$100–$275
A clogged condensate drain line backs up, trips the float switch, and shuts cooling down — on many systems it kills cooling entirely. Look for water around the indoor unit or a full drain pan. DIY half: wet-vac the outdoor drain stub. Recurring clogs, or no access to the line, is a professional job. Water pooling by the indoor unit may be a leak worth its own look.
9.Failed capacitor$170–$400
The outdoor unit hums but the fan won't spin — that's usually the capacitor, the most common electrical failure in an AC. Installed it runs $170–$400; the part alone is $15–$60 wholesale, and the spread covers the trip, diagnosis, warranty, and licensed labor. Do not open the panel: capacitors hold a charge after power-off. See the AC repair cost breakdown before you call.
The 5 technician-level causes (10–14)
From here down you're confirming, not fixing — sealed refrigerant circuits, high voltage, and system-level faults. Knowing the cause still shortens the visit and the bill.
10.Low refrigerant (a leak)$225–$1,600
Warm air, ice on the lines, and a hissing sound together point to refrigerant that left through a leak — and it will leave again after any top-up. A recharge without a leak repair is a subscription, not a fix. Refrigerant work is sealed-system, EPA 608–licensed only; there is no legal DIY. Confirm the symptoms on our low refrigerant signs page first.
11.Condenser fan motor failure$300–$700
The outdoor fan won't spin while the compressor runs, so the unit overheats and trips out on hot afternoons. The tell is cooling that works in the morning and dies mid-afternoon. This is a technician replacement.
12.Leaking or crushed ductwork$250–$1,500
The AC makes cold air but it escapes into the attic or crawlspace before it reaches you. Tells: some rooms cool and others don't, bills climb, and registers whistle. A duct pressure test confirms it — that's professional equipment, not a DIY check.
13.Undersized — or oversized — systemManual J
A system that never shuts off on a 95° day yet never reaches setpoint may be undersized. But oversized fails too: it short-cycles, skips dehumidification, and leaves the house clammy at 72°. Correct sizing is confirmed by a Manual J load calculation, not a rule of thumb — our AC sizing guide walks through it.
14.The system is just old (15+ years)See repair-vs-replace
Past 15 years, weak cooling is usually accumulated decline — worn compressor valves, micro-leaks, a tired blower — not one fixable part. The honest math: at this age, run the numbers on repair vs replace before paying for a refrigerant leak or a fan motor.
Which cause is it? Match your symptom
Find your exact symptom to narrow the suspects, then jump to the cause.
| What you're seeing | Likeliest causes | First move |
|---|---|---|
| Air fully warm from every vent | 1 · 5 · 10 | Check thermostat, then feel the outdoor unit's air |
| Air slightly cool but house won't drop | 2 · 7 · 13 · heat wave | Filter + condenser coil, then read the heat-wave section |
| One room warm, the rest fine | 3 · 12 | Walk that room's vents and returns |
| Cools, then quits mid-afternoon | 7 · 11 | Outdoor unit is overheating — check the fan and coil |
| Short bursts, on and off | 9 · 13 | Capacitor or a sizing/short-cycle fault |
| Water near the indoor unit | 8 | Clogged condensate drain line |
| Outdoor unit silent | 4 · 9 · 11 | Breaker/disconnect first, then capacitor or fan motor |
The symptom narrows the suspects; only a technician's gauge readings confirm the part.
The heat-wave reality check
On a 95–100°F+ day, a correctly working AC holds a 15–20°F difference between indoor and outdoor temperature. 78° inside when it's 100° outside is the system working, not failing.
Systems are sized to your local design temperature — a typical hot day for your climate — not the record high, so an extreme heat wave pushes even a healthy unit to its ceiling. High humidity adds a latent load on top, which is why a muggy 95° day feels worse than a dry one. If your AC is simply holding that 15–20° delta, nothing is broken; it's at capacity.
When to stop troubleshooting and call
Call when the breaker re-trips, ice returns after a full thaw, anything smells burnt, or you've cleared causes 1–8 with no change. Describe what you already checked — it shortens the visit and the bill. A typical repair visit runs $150–$650, and the $75–$200 diagnostic is often credited toward the repair.
Pricing the repair first? See AC repair cost. Want to know what happens on the visit? Read what to expect from a service call.
Get a licensed AC tech on the phone
One call routes you to a licensed local contractor for an AC that won't cool — 24/7, nationwide. The call is free.
☏ Call (888) 810-2291Availability is subject to provider participation, location, technician availability, and demand.
Keep it from happening again
- Swap the filter on schedule — a 1-inch filter monthly to quarterly, thicker media filters less often.
- Book a spring tune-up before the first heat — a real tune-up cleans the condenser, checks refrigerant charge, tests the capacitor, tightens electrical, and clears the drain line.
- Keep 2 ft of clearance around the outdoor unit and give it a gentle rinse once a year.
- Replace the thermostat batteries yearly so the whole system never freezes on a dead cell.
Frequently asked questions
Why is my AC running but not blowing cold air?
It splits two ways. If air moves from the vents but isn't cold, the problem is usually airflow or refrigerant — dirty filter, iced coil, dirty condenser, or a leak. If little or no air moves, or the outdoor unit is silent, it's electrical — a tripped breaker, a failed capacitor, or a fan motor. Feel the vents and check the outdoor unit to tell which.
Should I turn my AC off if it's not cooling?
Yes if you see ice anywhere on the unit or lines, or hear grinding — running it then strains the compressor and worsens the icing. Otherwise leave it running and work through the checks in order; most causes are safe to diagnose with the system on.
How do I reset my central AC?
Find the AC breaker in your electrical panel, switch it fully OFF, wait 30 seconds, then back ON; also check the outdoor disconnect box for a pulled fuse block. Reset only once. A breaker that re-trips is telling you there's a real electrical fault, and repeated resets are a fire risk, not a fix.
Why does my AC take so long to cool the house?
On a hot day a correctly sized system pulls the house down slowly — a few degrees an hour, not instantly. If it never catches up on a 95°+ day, it may be undersized or simply hitting the heat-wave capacity limit (see below), not broken.
One room won't cool — why?
Almost always a delivery problem local to that room: a blocked or closed supply vent, a return covered by furniture, or a leaking or crushed duct branch feeding it. Start by walking the vents and returns in that room before suspecting the whole system.
Do ceiling fans help when the AC can't keep up?
They help people, not rooms. A fan's breeze makes skin feel about 4°F cooler through wind chill, but it doesn't lower the air temperature — so run fans in rooms you're in and turn them off when you leave.
Does high humidity make the AC worse?
Yes. Humid air carries a latent load the AC has to remove before the room feels cool, so on muggy days the same system delivers less felt cooling for the same runtime. An oversized system makes this worse by short-cycling before it dehumidifies.
Does cleaning the filter make the AC colder?
If the filter was clogged, yes — it restores the design airflow across the coil, which stops the coil icing and lets cold air reach the vents. A clean filter can't make the air colder than the system's design, though.
Can I add refrigerant myself?
No. Handling refrigerant requires EPA 608 certification by law, and a low charge means there's a leak — adding more without repairing it just vents refrigerant again. This one is licensed-technician only. See our low refrigerant signs page to confirm the symptoms first.
Can a power surge break an AC?
Yes — a surge can take out the capacitor or the control board, which then shows up as a unit that hums but won't start or won't cool. A whole-home or dedicated HVAC surge protector is cheap insurance against it.