Why Is My AC Not Cooling? 9 Common Causes and How to Fix Them

When an AC runs but does not cool, the cause is usually airflow or maintenance — not a broken compressor. Start with the two fixes that solve most cases: replace a dirty air filter and make sure the thermostat is set to Cool, below room temperature. If those do not help, work down the list below from cheapest to most serious.

A quick rule of thumb: if warm air blows from the vents, the problem is usually cooling-related (refrigerant, a frozen coil, dirty coils). If little air moves at all, it is an airflow problem (filter, fan, vents) — or the unit may be the wrong size for the room. Use the BTU calculator to confirm your room is matched to the unit before assuming a fault.

This is the single most common cause. A filter caked with dust chokes airflow across the indoor coil, so the unit runs constantly but moves little cool air — and in bad cases the coil freezes over. Pull the filter out and hold it to the light; if you cannot see through it, clean or replace it.

Check the filter monthly during cooling season and replace (or wash, for reusable filters) every one to three months. It is a five-minute job that restores airflow, lowers your bill, and prevents most of the other problems on this list.

It sounds obvious, but a fan set to ON instead of AUTO blows uncooled air between cooling cycles, which feels like the AC is broken. Set the fan to AUTO, the mode to Cool, and the target a few degrees below the current room temperature. On a window or portable unit, confirm it is in Cool mode and not Fan or Dry mode.

If you have a smart thermostat, check the schedule — a setback you forgot about can let the room drift warm in the afternoon heat.

Furniture over a return vent, closed supply registers, or a dirty blower wheel all starve the system of air. Walk the room and make sure vents are open and unobstructed. For a window or portable unit, confirm the front intake and the rear (or exhaust hose) are not pressed against a wall or curtain.

Portable units are especially sensitive: a single-hose model pulls room air to cool its condenser and pushes it outside, creating negative pressure that draws hot air back in through gaps. A dual-hose portable avoids this and cools noticeably better in a hot room.

If you see ice on the coils or the copper line, the unit is iced up — and ice insulates the coil so it cannot absorb heat. The usual culprits are restricted airflow (see the filter and vents above) or low refrigerant. Turn the unit off and let it thaw fully, run the fan only to speed it up, then fix the underlying airflow problem before running it again.

If the coil freezes again within a day of thawing and the filter is clean, the cause is almost always low refrigerant — which needs a professional.

The outdoor unit (or the rear of a window or portable unit) sheds the heat your AC removes from the room. When those coils are matted with dust, grass clippings, or cottonwood fluff, the system cannot dump heat and the room never cools. Power the unit off and gently rinse the coils with a garden hose from the inside out; keep at least two feet of clear space around an outdoor condenser.

This is routine maintenance, not a repair — doing it each spring keeps efficiency up and is the cheapest way to protect a unit you already own.

An undersized AC cannot keep up on the hottest days — it runs nonstop and still loses ground. An oversized one is worse than it sounds: it blasts cold air, hits the set temperature in minutes, and shuts off before removing humidity, leaving the room cold but clammy and never running long enough to cool evenly.

If the room has always struggled to cool, the unit may never have been sized right. Our BTU calculator factors in square footage, climate, sun exposure, and insulation, and our sizing guide explains how to read the result.

Refrigerant is not consumed like fuel — if it is low, there is a leak. Signs include warm air from the vents, a hissing or bubbling sound, ice on the lines, and a system that runs forever without cooling. This is not a DIY fix: handling refrigerant requires EPA certification, and a leak has to be found and sealed before recharging.

If you suspect refrigerant, turn the unit off to avoid damaging the compressor and book a licensed HVAC pro.

If the indoor fan runs but the outdoor compressor is silent, a tripped breaker, blown fuse, or failed capacitor may have taken the compressor offline. Check the breaker panel and reset once; if it trips again immediately, stop — a repeatedly tripping AC circuit points to a real electrical fault that needs a technician.

For window and portable units, plug the unit straight into a dedicated wall outlet, not a power strip or extension cord, which can drop voltage and trip the unit's own breaker.

Call a pro when the basics check out but the unit still will not cool: anything involving refrigerant, a compressor that will not start, electrical faults that trip the breaker, or a coil that keeps freezing after the filter and airflow are clean. These are not safe or legal to DIY, and a cheap diagnostic visit often prevents a far more expensive failure.

You can find a top-rated HVAC contractor in your state in our directory — and if the unit is old and the repair is expensive, our buying guides and comparison tool will help you choose a more efficient replacement.