Why Is My AC Running but Not Cooling?

The short answer: an AC that runs without cooling usually has one of four problems — a clogged filter starving the system of airflow, a frozen evaporator coil, a failed capacitor keeping the outdoor unit from starting, or a refrigerant leak. Check the filter and the outdoor unit yourself first; the last two need a certified technician with gauges.

1. Start with the filter

Your air conditioner doesn’t make cold air out of nothing — it pulls heat out of the air moving across its indoor coil. A packed filter chokes that airflow, so less warm air reaches the coil, less heat gets removed, and the system runs longer and longer while the house barely budges. Pull the filter and hold it up to a light. If you can’t see light through it, replace it and give the system an hour. This is the cheapest fix in HVAC, and it’s the true cause of a surprising share of “my AC is broken” calls.

2. Look for ice on the lines

Low airflow — or low refrigerant — can drive the evaporator coil below freezing. Moisture in the air then freezes onto the coil, and the ice itself blocks even more airflow, so the problem snowballs. Look at the copper lines where they enter the indoor unit, or at the coil itself if you can see it. Frost or ice means: turn the cooling off, set the fan to ON, and let it thaw completely — usually a few hours. Running a frozen system risks flooding the drain pan and, worse, sending liquid refrigerant back to the compressor. If the ice comes back after a thaw and a fresh filter, the underlying cause needs a diagnosis, not another thaw.

3. Is the outdoor unit actually doing anything?

Go stand next to the condenser — the outdoor unit. You should hear the compressor humming and feel warm air blowing out the top. If the fan spins but the air isn’t warm, or the unit hums and clicks without really starting, a run capacitor is a prime suspect. Capacitors store the electrical kick that starts the compressor and fan motors, and they’re one of the most common failure parts in cooling season — heat kills them. The fix is quick for a technician on an AC repair call, but capacitors hold a charge even with power off, so this is not a screwdriver-and-YouTube repair.

While you’re out there: clear leaves, cottonwood fluff, and grass clippings off the coil fins, and keep a couple of feet of open space around the unit. A blanketed condenser can’t dump heat, and the whole system suffers for it — a spring AC tune-up includes clearing and cleaning that coil.

4. Refrigerant leaks — and why this one isn’t DIY

Refrigerant doesn’t get “used up.” A system that’s low on charge has a leak, full stop. The symptoms overlap with everything above — long run times, ice on the lines, air from the vents that’s cool but not cold. Finding the leak takes gauges, electronic detectors, and sometimes dye; fixing it right means repairing the leak, pulling a vacuum, and recharging to the correct weight.

One more thing worth knowing: under Section 608 of the Clean Air Act, handling refrigerant legally requires EPA 608 certification. That’s not marketing — it’s federal law, and it exists because vented refrigerant is an environmental problem. Anyone offering to “top off” your system without finding the leak is selling you the same failure again next summer.

When to stop troubleshooting and call

You’ve replaced the filter, thawed any ice, cleared the outdoor coil, and the house still won’t cool — that’s the point where the diagnosis needs meters, gauges, and training. We’ll test the capacitor, measure the refrigerant charge, find the actual fault, and show you the evidence before quoting the fix. If you’re sweating through a Chicago heat wave while you read this, we answer 24/7.