AC Running But Not Cooling? 8 Causes and How to Fix Them

It is 4 p.m. on a Tuesday in August. The thermostat reads 84, the unit outside is humming, the indoor blower is moving air, and nothing is getting cold. This is the call I get more than any other in summer, and about half the time it is something the homeowner can fix in 10 minutes without a tool more complicated than a flashlight.

Below are the eight causes in the order I check them, fastest and cheapest first. Work through them in this sequence and you will find most problems before you ever pick up the phone.

1. Thermostat Set Wrong (5 minutes)

I have driven 45 minutes for service calls that turned out to be a thermostat set to FAN ON instead of AUTO. When the fan is set to ON, the blower runs continuously even when the compressor is off, blowing room-temperature air through the vents between cooling cycles. Homeowners feel warm air and assume the AC is broken.

Check that the mode is set to COOL (not HEAT, not FAN ONLY) and the fan is set to AUTO. Set the temperature at least 4 degrees below the current room reading so the system has a reason to run. If it is a battery-powered thermostat, swap the batteries. A weak battery causes flaky behavior that mimics a dead system.

2. Dirty Air Filter (5 minutes)

A clogged filter is the leading cause of "AC not cooling" calls I run. The blower cannot pull enough air across the evaporator coil, which causes one of two things: the coil freezes into a block of ice and stops absorbing heat, or the system runs constantly with very weak cold output.

Pull the filter and hold it up to a light. If you cannot see the light bulb clearly through the filter, replace it. Most homes need a new filter every 1 to 3 months in summer. If you find ice on the refrigerant lines or on the indoor coil, turn the system off at the thermostat, set the fan to ON, and let it thaw for 2 to 4 hours before restarting.

3. Frozen Evaporator Coil (1 to 4 hour fix)

If the indoor coil is iced over, the system cannot transfer heat and the cold air stops. Common causes:

• Dirty air filter (see above)
• Blocked or closed supply vents reducing airflow
• Dirty indoor coil (needs professional cleaning)
• Low refrigerant from a leak
• Failed blower motor or capacitor

To thaw safely: turn the thermostat to OFF and the fan to ON. Wait at least 2 hours, longer if you see significant ice. Once everything is dry, replace the filter, reopen any closed vents, and restart in COOL. If it freezes up again within 24 hours, you have an underlying problem (low refrigerant or a dirty coil) that needs a pro.

4. Bad Run Capacitor (Pro Job, $150 to $400)

This is the failure I see more than any other on residential AC units. The dual run capacitor is a little metal can inside the outdoor unit that gives the compressor and outdoor fan the kick they need to start. When it fails, you get a classic symptom: the unit hums but the big fan on top doesn't spin (or only spins if you give it a nudge with a stick, which is a quick field test, not a fix).

Insider tip: Most run capacitors are rated for 5 to 10 years. If yours is 10+ years old, replacing it preventively during your spring tune-up costs $150 to $250 and almost always prevents a July emergency call at 1.5x rates. I always check the microfarad reading on customers' capacitors during maintenance, even if the system is running fine.

Do not try this yourself unless you know how to discharge a capacitor. They store dangerous voltage even with power off.

5. Dirty Condenser Coil (1 hour, DIY-friendly)

The outdoor unit dumps the heat your house generates into the outside air. If the outdoor coil (the fins on the sides of the unit) is clogged with cottonwood fluff, grass clippings, or dog hair, it can't release heat, and your indoor air stops getting cold.

Safe homeowner cleaning:

1. Shut off power at the disconnect box on the wall next to the unit (the gray box). Then flip the breaker too.
2. Use a soft brush or a shop vac to clear loose debris from the fins.
3. Rinse from the inside out with a garden hose on a gentle setting. Do not use pressure washer, you will flatten the aluminum fins.
4. Trim back any plants within 2 feet of the unit. The condenser needs airflow on all sides.
5. Wait 15 minutes for everything to dry, restore power, and restart the system.

A heavily clogged coil can drop capacity 20 to 40 percent. I have seen homes drop 5 degrees indoors within an hour of a thorough coil rinse.

6. Low Refrigerant from a Leak (Pro Job, $400 to $1,500)

Refrigerant does not get "used up" the way fuel does. It circulates in a sealed loop. If your system is low, you have a leak. Common signs:

• Vents are blowing air but it is barely cool, especially after a hot afternoon
• Ice on the copper refrigerant lines or on the indoor coil
• A hissing or bubbling sound near the indoor or outdoor unit
• Higher-than-normal electric bills with worse-than-normal cooling

A technician will pressurize the system with nitrogen, find the leak (often with electronic leak detectors or UV dye), repair it, evacuate the system with a vacuum pump, and recharge to the manufacturer's spec. Just topping off without finding the leak is a waste of money. As of 2026 the most common refrigerants in residential systems are R-410A (in systems built 2010 to 2024) and R-32 or R-454B (in systems built 2025+, due to EPA AIM Act refrigerant transitions). If your system uses R-22 (built before 2010), refrigerant alone runs $80 to $150 per pound and a leak repair is rarely worth it.

7. Tripped Breaker or Blown Fuse (5 minutes)

Walk to your main electrical panel and the outdoor disconnect box. AC systems typically have two breakers in the main panel (240V double pole) for the outdoor unit and one for the indoor air handler. If a breaker is tripped, flip it all the way OFF, then back to ON.

If it trips again within minutes, stop. A breaker that keeps tripping means a real electrical problem (shorted compressor, locked rotor, failed fan motor) and resetting it repeatedly can damage the system or start a fire. Call a pro.

8. Failed Compressor (Pro Diagnosis, Usually Replacement)

The compressor is the heart of the system. When it fails, the outdoor unit may hum briefly, click, and shut off. Or it may run continuously but the lines stay warm. On any system over 10 years old, a failed compressor usually means it is time to replace the system rather than the compressor. Compressor replacements cost $1,200 to $2,800 and the rest of the aging system is right behind it.

Use our free HVAC age lookup to check your system's manufacture date and our repair vs replace guide to run the math.

Quick Decision Reference

Symptom	Most Likely Cause	Action
Weak airflow + warm air	Dirty filter or frozen coil	Replace filter, thaw 2-4 hours
Outdoor unit humming, fan still	Failed capacitor	Call HVAC tech ($150 to $400)
Outdoor unit silent	Tripped breaker or contactor	Check breakers, then call pro
Ice on refrigerant lines	Low refrigerant or airflow	Thaw, replace filter, then pro
Cool air, just not cold enough	Dirty condenser coil	Rinse outdoor coil with hose
House warm, vents blowing room temp	Thermostat on FAN ON	Switch fan to AUTO

When to Stop DIY and Call a Pro

Call a licensed HVAC tech if:

• The breaker trips more than once after a reset
• You see ice that returns within a day of thawing
• You hear hissing, gurgling, or smell anything like burning insulation
• Your system is over 10 years old and has multiple problems in one season

For finding reputable contractors, the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) certifies equipment matches, and most reputable contractors carry the NATE technician certification. Ask before scheduling.

Prevent the Next Failure

Most no-cool calls come down to skipped maintenance. Annual spring tune-ups are the cheapest insurance you can buy on an AC system. Follow our HVAC maintenance checklist to stay ahead, and read why your AC blows warm air if your problem is closer to no cold air at all rather than weak cold air. The ENERGY STAR maintenance checklist is another solid reference.