AC Running but Not Cooling? What to Check, in Order

An AC that runs but doesn't cool usually has a clogged filter, an iced coil, a failed capacitor or a refrigerant leak, and a thermometer in the supply and return registers tells you which kind of problem you have. Work the checks below in the order a technician would, easiest first. In Minnesota the condenser sat idle all winter, so weak parts tend to quit in the first real heat.

What you see, and what it usually means

Is your AC broken, or just falling behind on a hot day?

After the system has run for 15 minutes, hold a kitchen thermometer in a supply register near the furnace, then in the return grille. A working central AC usually cools the air roughly 15–20 degrees on its way through. If you're seeing that and the house is still warm, the AC is working and losing the race. That happens on afternoons hotter than the system was designed for, in homes with a lot of west-facing glass, and in story-and-a-half houses where the upstairs never had much ductwork. Our post on uneven home temperatures covers those.

If the split is small, or the air from the registers is room temperature, something is wrong. Keep going.

Checks you can make in 20 minutes

1. Thermostat. Mode on Cool, fan on Auto (not On), setpoint a few degrees below room temperature. On a smart thermostat, look for a schedule, a hold, or a utility event message.
2. Filter. Pull it and look. If it's gray and matted, replace it. A starved coil can freeze, and ice blocks the rest of the airflow.
3. Ice. Look at the larger copper line where it enters the indoor coil and at the outdoor unit. Frost or ice means low airflow or low refrigerant, so turn cooling off and run the fan until it melts. Our guide to why an AC freezes up has the full thaw steps and what to check before you restart.
4. Outdoor unit. With the system calling for cooling, go outside. The fan on top should be spinning and you should hear the compressor. If the fan is stopped and the unit hums, turn it off at the thermostat; that's usually a capacitor or fan motor, covered in our post on AC humming noises. If the outdoor unit is dead silent, check the breaker and the disconnect box on the wall next to it.
5. Outdoor coil. Cottonwood fluff in June, grass clippings and dryer lint can blanket the fins. With the power off at the disconnect, rinse from the inside out with a garden hose, never a pressure washer. Our outdoor coil cleaning guide shows how.
6. Condensate. Look around the furnace for water or a full drain pan. A float switch can shut cooling off when the drain backs up, and our post on an AC that won't turn on covers that along with the breaker and disconnect checks. The drain itself is in our guide to clearing a clogged condensate drain.

Minnesota patterns worth knowing

Utility cycling and shared pumps. Xcel can cycle an enrolled AC during peak demand, and in many basements the AC and a high-efficiency furnace share one condensate pump. Both can make a healthy system look dead, and both are covered in the guide linked in step 6.

Smoky days. When wildfire smoke drifts down from Canada, filters load faster than usual. A filter that looked fine two weeks ago can be choking the coil after a smoky stretch, so check it again.

Muggy nights. On a July night with a high dew point, a large share of the AC's effort goes into removing moisture, so the temperature falls slowly. A system that holds 74 and keeps the house dry is doing its job.

How do you know an AC is low on refrigerant?

An AC doesn't use up refrigerant, so if it's low, it leaked. The signs are ice on the lines, a small temperature split, a hissing sound near the indoor coil and a system that slowly got weaker over several summers. The right repair is to find the leak, fix it, then charge the system, because topping it off without finding the leak buys a few weeks. A leaking indoor coil is a common source, and our post on evaporator coil leak symptoms shows what that looks like. Our refrigerant recharge cost guide covers pricing.

An R-22 system with a leak is usually the point to price replacement, as our R-22 options guide explains. Existing R-410A systems can still be repaired and recharged. Nothing in the EPA refrigerant transition requires you to replace a working R-410A unit.

What happens on an AC repair visit?

A service call traces the fault through the electrical parts, airflow, drain or refrigerant side, as the symptoms point, and you get a written flat-rate price before any work starts. Completed repairs carry a written 1-year parts-and-labor warranty. For scale on the common fixes, HomeGuide lists a capacitor at $150–$400, a contactor at $200–$450 and a condensate drain flush at $75–$250 nationally. Those are national ranges, not our prices; ours comes as a flat quote once the fault is found.

When should you turn off an AC that isn't cooling?

Some problems get worse every minute the AC runs. Turn cooling off at the thermostat and call if:

• The outdoor fan is stopped while the unit hums.
• Ice comes back within a day of thawing.
• The breaker trips a second time after one reset.
• You hear hissing at the indoor coil, or see oily spots on the refrigerant lines or fittings.
• You smell burning from the furnace or the outdoor unit.

What to do next

If the coil iced, the outdoor fan won't spin, or the breaker trips, turn the AC off and leave it off so a small problem doesn't become a compressor. Then call 612-429-3465. We're open 7 days, 7am–9pm, and our AC repair page covers what a visit includes. The price is the same across our service area, from AC repair in Minneapolis to AC repair in Saint Paul and the towns around them.

Common questions about cooling

Sources

• U.S. Environmental Protection Agency epa.gov
• HomeGuide: AC repair costs homeguide.com