Why is my air conditioner running but not cooling the house?
An AC that runs but won't cool usually has a clogged filter or iced coil, a dirty outdoor coil, a failed capacitor or fan motor, or a refrigerant leak. Check the thermostat, filter, breaker and outdoor fan first. If the outdoor fan is stopped or the lines are iced, turn it off and call.
Key points
- Measure the air at a supply register and the return: roughly 15–20 degrees cooler means the AC works but is falling behind.
- Thermostat, filter, breaker, outdoor fan, outdoor coil and condensate drain cover most homeowner fixes.
- Ice on the lines or a stopped outdoor fan: turn the AC off to protect the compressor.
- Low refrigerant means a leak; it should be found and fixed before recharging.
- Our $79 trip charge is waived when you approve the repair, and the price is written down first.
When an air conditioner runs but the house won't cool, the cause is usually one of five things: a clogged filter that has iced the indoor coil, a dirty outdoor coil, a failed capacitor or fan motor in the outdoor unit, a backed-up condensate drain, or low refrigerant from a leak. You can spot the first four yourself in about 20 minutes. The checks below run in the order a technician works through them, easiest and most common first, and they'll either fix it or tell you exactly what to say on the phone.
What you see, and what it usually means
| What you find | Likely cause | What to do | Who does it |
|---|---|---|---|
| Registers blow air 15–20 degrees cooler than the return, house still warm | AC works but is falling behind | Close blinds on sunny sides, keep interior doors open | You; a technician for duct or sizing problems |
| Gray, matted filter | Low airflow, often with an iced coil | Replace the filter, thaw the coil if iced | You |
| Frost or ice on the copper line or indoor coil | Low airflow or low refrigerant | Cooling off, fan On, thaw for several hours | You, then a technician if it refreezes |
| Outdoor fan stopped, unit humming | Failed capacitor or seized fan motor | Turn cooling off | Technician |
| Outdoor unit silent, indoor fan running | Breaker, disconnect, utility event, wire or contactor | Check the breaker and disconnect once | You, then a technician |
| Water around the furnace or a full pump | Clogged condensate drain or failed pump | Clear the drain, check the pump | You or a technician |
| Hissing at the indoor coil, cooling faded over seasons | Refrigerant leak | Turn it off and book a leak search | Technician |
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
- 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.
- 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.
- 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. Turn cooling off, set the fan to On, and give it a few hours to melt completely before trying again. If it ices up again within a day, the filter wasn't the whole story.
- 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.
- 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.
- Condensate. Look around the furnace for water, a full drain pan or an overflowing condensate pump. Many systems have a float switch that shuts cooling off when the drain backs up. See our guide to clearing a clogged condensate drain.
Minnesota patterns worth knowing
The first hot week. The condenser sat from fall to late spring. Capacitors and fan motors that were weak last year tend to quit in the first real heat.
One pump, two jobs. In most Minnesota basements the AC coil sits on top of the furnace. If you have a high-efficiency furnace, its condensate and the AC's often share the same pump and line. A pump that handled furnace water all winter can quit in its first week of summer duty, and the line can clog with sludge.
Utility cycling. If you're an Xcel Energy customer enrolled in Saver's Switch or AC Rewards, Xcel can cycle your AC during peak summer demand. During an event you may hear the indoor blower while the outdoor unit sits idle, and a smart thermostat will usually show the event. Our AC Rewards review explains how those programs work.
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. If it's low, it leaked. The signs: ice on the lines, a small temperature split, a hissing sound near the indoor coil, and a system that ran fine for years and slowly got weaker. The right repair is to find the leak, fix it, then charge the system. Topping it off without finding the leak buys a few weeks.
National cost guides give a sense of scale. HomeGuide puts leak search and repair with recharge at $200–$1,500, depending on where the leak is. R-410A runs about $50–$80 per pound installed in HomeGuide's figures, while R-22, which ended production and import in 2020, runs $90–$150 per pound for reclaimed or stockpiled supply. A leaking indoor coil is a common source; our post on evaporator coil leak symptoms shows what that looks like, and our refrigerant recharge cost guide covers pricing.
An R-22 system with a leak is usually the point to price replacement, and our AC replacement cost guide shows what that involves. 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 checks the electrical parts, the airflow, the drain and the refrigerant side, and you get a written flat-rate price before any work starts. The $79 trip charge is waived when you approve the repair, and 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, or you can book AC repair online. 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.
Questions
Is it normal for my AC to fall behind on a 95-degree day?
Yes, to a point. Central air is sized for a design temperature, and on afternoons hotter than that the house can drift a few degrees above the setpoint and catch up after sunset. If it falls behind on 80-degree days, or the air at the registers isn't roughly 15–20 degrees cooler than the return air, something needs attention.
How long should I let a frozen AC thaw?
Until the coil and lines are completely clear of ice and the water has drained, with cooling off and the fan running on On. That often takes several hours. Replace the filter before restarting. If the coil freezes again within a day, the cause is low refrigerant or an airflow problem that needs a technician.
Can I add refrigerant to my AC myself?
No. Handling refrigerant requires EPA Section 608 certification, and an AC that's low has a leak. Adding refrigerant without finding the leak means paying for it again soon, and overcharging can damage the compressor. A technician should find and repair the leak, then charge the system by weight and measurement.
Should I replace an old AC that isn't cooling?
It depends on age, refrigerant and the failed part. A capacitor or contactor is usually worth fixing on almost any unit. A refrigerant leak on an R-22 system, or a failed compressor on a unit past 15 years, is usually the point to price replacement. We give a written repair price so you can compare.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com
- U.S. Environmental Protection Agency epa.gov