AC Running but Not Cooling? 7 Causes Houston Techs Check First

Published October 16, 2026 · Reviewed October 16, 2026

Part of our AC Repair & Service guides.

What does "AC running but not cooling" actually mean?

"Running but not cooling" means the system is moving air through your vents, but that air is not cold enough to pull the house down to the thermostat setting. Sometimes the indoor fan runs while the outdoor unit sits silent. Sometimes both run and the air still comes out cool-ish instead of cold. Those are different problems with different causes, so the first job is to figure out which one you have.

A central air conditioner has two halves. The indoor air handler or furnace blows air across the evaporator coil. The outdoor condenser holds the compressor and a fan that dumps the heat your house collected. Refrigerant moves heat between the two. If any link in that chain is weak, you get airflow without real cooling.

The quickest diagnostic question is simple: is the outdoor unit running when the thermostat calls for cooling?

When does this problem usually show up in Houston?

It shows up most on the hottest stretch of summer, because that is when an air conditioner works closest to its limit. At Houston Intercontinental Airport (IAH), the 1991–2020 normal high is 94.5°F in July and 94.9°F in August, with roughly 111 days a year at or above 90°F. A system that is slightly low on capacity, dirty or poorly matched to the house can keep up in May and fall behind in August.

That timing matters for diagnosis. A system that cools well at night but loses ground every afternoon may be working correctly but undersized for peak heat, or it may be losing capacity to a dirty coil or a slow refrigerant leak. A system that suddenly stops cooling on a mild day points more toward a failed part or a tripped safety.

What are the seven most common causes?

Most "running but not cooling" calls trace back to one of these seven causes, roughly in the order a technician rules them out.

1. Thermostat set wrong. The system is set to "Heat," "Fan On" or "Auto" with a setpoint above room temperature, or a schedule is overriding your change. With the fan set to "On," the blower runs nonstop even when the compressor is off, so you feel moving air that is not cold.
2. Clogged air filter. A filter packed with dust starves the indoor coil of airflow. Less air across the coil means less cooling at the vents and, over time, a coil that can freeze.
3. Outdoor unit has no power. A tripped breaker, a pulled outdoor disconnect or a failed electrical part can leave the indoor fan running while the condenser does nothing.
4. Dirty outdoor condenser coil. Grass clippings, dirt, pollen and leaves block the fins, so the unit cannot shed heat. Cooling drops as the outdoor temperature climbs.
5. Frozen evaporator coil. Low airflow or low refrigerant can let the indoor coil drop below freezing and ice over. Ice blocks airflow, and vent air gets weaker and warmer.
6. Low refrigerant from a leak. Refrigerant is not used up like fuel. If the charge is low, it leaked out somewhere, and the system loses capacity until the leak is found and repaired.
7. Failed capacitor, contactor or compressor. The capacitor gives the compressor and outdoor fan motor their starting boost. When it fails, you may hear a hum at the outdoor unit with no fan spinning, or nothing at all.

Ductwork can also be the culprit. A disconnected or crushed duct in the attic can dump cold air where nobody feels it, which shows up as one or two weak rooms rather than a whole house that won't cool.

What can you safely check yourself?

You can safely check the thermostat, the air filter, the breakers and the area around the outdoor unit. Work through these in order before you call anyone.

1. Thermostat. Confirm it says "Cool," set it a few degrees below room temperature and set the fan to "Auto." If the screen is blank, see our post on how to tell whether your thermostat has power.
2. Air filter. Pull it and hold it up to a light. If you can't see light through it, replace it with the same size.
3. Breakers. Look for a tripped breaker labeled for the AC, condenser or air handler. Reset a tripped breaker once. If it trips again, leave it off and call a licensed technician, because a repeat trip means an electrical fault.
4. Outdoor unit. With the system calling for cooling, check whether the outdoor fan is spinning. Clear plants, debris and anything stacked against it. Our guide on how to clean a Goodman AC unit covers gentle coil rinsing with the power off.
5. Signs of ice. Look at the refrigerant line at the outdoor unit and, if you can see it, the indoor coil. If you see ice, switch the system to "Off" and the fan to "On" to thaw it. Do not chip at the ice.

If the filter was the problem, cooling should improve within an hour or two once the coil has thawed and airflow is back. If nothing on this list explains it, the cause is almost always inside the equipment.

What needs a licensed technician?

Anything involving refrigerant, electrical components or the compressor needs a licensed technician. In Texas, air conditioning work is licensed by the Texas Department of Licensing and Regulation (TDLR), and refrigerant handling requires EPA certification.

• Refrigerant. Checking pressures, finding a leak and recharging require gauges, certification and a leak repair. Topping off without a repair just postpones the next failure. For the warning signs, read how to know if your AC is low on Freon.
• Capacitors and contactors. A capacitor can hold a dangerous charge even with the power off. Our post on the capacitor in a Goodman AC explains what the part does and why it fails; leave the replacement to someone trained to discharge and test it.
• Compressor, fan motors and control boards. These require electrical testing to diagnose correctly.
• Repeated freezing. If the coil ices again after a new filter, the root cause is airflow or charge, and both need instruments to measure.

A rule worth keeping: if a fix involves opening a panel with a warning label, it is a job for a licensed technician.

How can you tell if a fix worked?

A fix worked if the outdoor unit runs whenever the thermostat calls for cooling, the vent air feels noticeably colder than room air, and the house reaches the setpoint and holds it. Watch the system through one full afternoon, since that is when Houston heat exposes a marginal system.

• Note how long the system runs and whether it shuts off after reaching temperature.
• Check the indoor temperature at the warmest part of the day, not just in the morning.
• Listen to the outdoor unit for humming without a spinning fan, clicking or rattling.
• Check the filter again after a week if it was very dirty, so you know how fast your home loads it.

If the house still won't reach temperature, write down what you saw: whether the outdoor unit runs, any ice, any breaker trips, and when the problem shows up. That record shortens the diagnosis.

What should you do next?

If the safe checks didn't solve it, schedule a diagnosis rather than running the system hard and hoping. A system that runs without cooling wastes electricity, and a frozen coil or a struggling compressor can turn a modest repair into a bigger one.

When the technician gives you a diagnosis, ask what failed, why, and whether the same issue is likely to come back. If the system is older or the repair involves refrigerant or the compressor, that is the point to weigh repairing against replacing, using the system's age, its refrigerant type and the repair quote in front of you.

Frequently Asked Questions

Why is my AC blowing air but not cold air?

The most common reasons are a thermostat set to "Fan On," a clogged air filter, an outdoor unit that has lost power, or a frozen indoor coil. Less common but more serious causes are low refrigerant from a leak and a failed capacitor or compressor. Check the thermostat, filter and breakers first; the rest need a licensed technician.

Should I turn my AC off if it's not cooling?

Yes, if you see ice on the refrigerant lines or indoor coil, or if the outdoor unit is humming without the fan spinning. Running it in those conditions can damage the compressor. If you see ice, set the system to "Off" and the fan to "On" to thaw the coil while you arrange service.

Can a dirty air filter really stop an AC from cooling?

Yes. A clogged filter cuts airflow across the indoor coil, which reduces cooling and can let the coil freeze. Replacing the filter is the cheapest and safest first step, and it fixes more "not cooling" complaints than most people expect.

Why does my AC cool at night but not during the afternoon?

Afternoon is when the load peaks. Houston's normal July and August highs are close to 95°F at IAH, so a system that is dirty, slightly low on charge or undersized for the house can keep up overnight and fall behind by mid-afternoon. A technician can measure whether the system is delivering its rated capacity.

Can I add refrigerant to my AC myself?

No. Handling refrigerant requires EPA certification, and a low charge means there is a leak that needs to be found and repaired. Adding refrigerant without fixing the leak only delays the next breakdown.

Is it worth repairing an AC that isn't cooling?

It depends on what failed, the system's age and its refrigerant type. A filter, breaker or capacitor issue is usually a straightforward repair. A compressor failure or major leak on an older system is when a repair-versus-replace comparison is worth doing.

Sources

1. NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, Houston Intercontinental Airport (USW00012960) — July and August normal highs and days at or above 90°F (as of October 16, 2026)
2. ENERGY STAR (U.S. EPA), HVAC Maintenance Checklist — role of filters, coils and fins in cooling performance (as of October 16, 2026)
3. U.S. EPA, Section 608 of the Clean Air Act: Stationary Refrigeration and Air Conditioning — certification requirement for technicians who handle refrigerant (as of October 16, 2026)
4. Texas Department of Licensing and Regulation, Air Conditioning and Refrigeration program — state licensing of air conditioning contractors (as of October 16, 2026)