AC Running but Not Cooling: Common Causes and When to Call for AC Repair in San Antonio

Check Thermostat and Basic Settings

This sounds too simple, but thermostat settings and basic power issues cause a lot of “AC is running but not cooling” calls. Here’s why: the indoor fan can move air even when the system isn’t in a true cooling cycle. So you feel airflow and assume cooling is happening—when it may not be.

Start with the “3 checks”:

• Mode: Make sure it’s set to Cool (not Heat, Off, or Fan).
• Fan setting: Set the fan to Auto, not On. “On” runs the blower nonstop and can make the home feel warmer and stickier between cycles because it keeps moving air across a damp coil.
• Setpoint test: Lower the temperature 3–5 degrees and listen. You should typically hear the indoor unit start and then the outdoor unit start shortly after.

Then do a quick thermostat sanity check:

• Battery-powered thermostat? Replace the batteries. Weak batteries can cause random behavior or incomplete calls for cooling.
• Thermostat location: If it’s in direct sunlight or near a hot kitchen, the reading may not reflect the rest of the home. That won’t usually cause “no cooling,” but it can lead to comfort problems.
• Schedule/hold settings: Double-check that a schedule didn’t reset your temperature higher than you expect.

Now check power to the system:

• Breaker panel: Look for a tripped breaker labeled AC, condenser, furnace, air handler, or HVAC. Reset it once. If it trips again, stop and call a technician.
• Indoor shutoff switch: Many indoor units have a light-switch-style shutoff near the air handler/furnace. If it was bumped off, the system may “run” in a limited way without cooling correctly.
• Indoor unit door: Some systems have a safety switch that won’t allow operation if the blower compartment door isn’t seated properly (common after filter changes).

What this tells you: If the thermostat is clearly calling for cooling but the outdoor unit never starts, you’re likely dealing with an outdoor power issue or an electrical-start problem (see Section 7).

Confirm Airflow Basics (Vents, Returns, Doors)

Before you assume a major mechanical failure, confirm your system can move air through the house. Cooling isn’t just “making cold air.” It’s a loop: warm air must get pulled back to the system (return air), cooled, and delivered back into rooms (supply air). Break that loop, and comfort drops fast.

Do this quick walk-through (5 minutes):

• Open supply vents: Make sure multiple supply registers aren’t shut. Closing too many vents can reduce airflow and lead to icing.
• Check the return grille: If the main return is blocked by a couch, rug, or drapes, airflow suffers. The system can run all day and still struggle.
• Replace “return filters” only where intended: Some homes have filters at the return grille; others have them at the air handler. Using both (double filtering) can choke airflow.
• Interior doors: If you keep bedroom doors closed, make sure the room has a return path (a return vent or enough under-door clearance). A “sealed” room can reduce overall airflow balance and make some rooms hotter.

What this tells you: If airflow is weak everywhere, the issue is often at the system (filter, iced coil, blower problem). If airflow is strong in some rooms and weak in others, duct or balancing issues become more likely.

Replace a Clogged Air Filter (and Restore Airflow)

Airflow is the foundation of cooling. Your AC needs to pull warm indoor air across the evaporator coil, remove heat and humidity, then push cooled air back through your home. When airflow is restricted, the system can’t move enough air to do that job—even if the equipment is “running.”

What a dirty filter can cause:

• Rooms that won’t cool evenly (some vents feel weak)
• Long run times with little temperature drop
• Ice on the coil or refrigerant line (next section)
• Higher energy bills because the system works harder

Quick test: pull the filter and hold it up to a light. If you can’t see light through it, replace it.

Do it the right way:

• Match the size: the filter size is printed on the frame (for example, 16x25x1).
• Check airflow direction: install it with the arrow pointing toward the unit (toward the return duct/air handler).
• Don’t “upgrade” too far: high-filtration filters can reduce airflow on some systems. If you want better filtration, ask a technician what your system can handle—or consider a dedicated indoor air quality upgrade instead of over-restricting the filter.

How often should you change it? Many homes do well with every 30–90 days. But pets, construction dust, allergies, and heavy summer run time can push you toward the shorter end.

Pro tip: Write the install date on the filter frame. It takes the guesswork out of it.

What this tells you: If a new filter noticeably improves airflow but the air still isn’t cold after 30–60 minutes, move to the ice check next.

Look for Ice on the Indoor Coil (and Thaw It the Right Way)

It feels backward, but your AC can freeze up even when it’s 100° outside. When airflow is too low—or when refrigerant is low—the evaporator coil can drop below freezing. Moisture in the air turns into ice on the coil. Once ice builds up, airflow gets even worse. That’s why an iced coil often starts as “it’s not cooling enough,” then turns into “it’s blowing warm air.”

Common signs of icing:

• Frost or ice on the copper line near the indoor unit
• Very weak airflow at the vents
• Water around the indoor unit after the ice melts
• A return vent that seems to “whistle” because the system is starved for air

Where to look (without taking anything apart):

• At the indoor unit: the refrigerant line entering the cabinet may show frost
• At the outdoor unit: the larger insulated copper line may feel unusually cold or show sweating/frost

What to do (safe thaw process):

• Turn the thermostat from Cool to Off.
• Set the fan to On to move warmer indoor air across the coil.
• Replace the air filter (if you haven’t already).
• Give it time. A fully frozen coil can take hours to thaw—sometimes up to 24 hours.

What not to do:

• Don’t chip at the ice with tools (a punctured coil can turn into a major repair).
• Don’t pour boiling water on components or try to use a high-heat device inside the cabinet.
• Don’t keep running the AC in Cool while it’s iced (you can damage the compressor).

What this tells you: If the coil freezes again soon after thawing and replacing the filter, the problem is usually not “just the filter.” It can be duct restriction, a blower issue, or a refrigerant leak. That’s a strong sign to schedule service.

Clean the Outdoor Condenser (Especially After Pollen and Storms)

The outdoor unit has one main job: release heat. If it can’t get rid of heat efficiently, your AC can’t cool well—especially during peak afternoon temperatures in San Antonio.

In South Texas, it doesn’t take much to block airflow through the condenser coil: grass clippings, dust, cottonwood, and seasonal pollen. After a storm, you may also see leaves or mud stuck to the coil surface.

What to check first:

• Is the outdoor fan spinning? If it’s not, don’t ignore it—running a compressor without proper airflow can cause damage.
• Is the unit clear? Aim for about two feet of clearance around all sides.
• Do you see buildup? If the fins look matted with debris, heat can’t escape well.

Safe rinse basics:

• Turn off power at the disconnect or breaker.
• Use a garden hose with gentle water pressure (never a pressure washer).
• Rinse from top to bottom to wash debris down and away.
• If the fins are bent or crushed, don’t try to “fix” them with random tools—ask a technician about proper fin combing.

Common mistake: Don’t pile mulch or landscaping up around the unit. It’s great for the yard, but it reduces airflow and invites debris into the coil.

What this tells you: If cleaning the outdoor unit improves cooling, plan on regular maintenance before the hottest part of the season. If nothing changes, move on to refrigerant or electrical possibilities.

Rule Out Refrigerant Leaks (It’s Not a “Refill” Problem)

Refrigerant doesn’t get “used up.” Your AC is a closed loop. So when a system is low on refrigerant, the real issue is usually: there’s a leak. The right fix is to find the leak, repair it, and then charge the system to the correct level.

What low refrigerant can look like at home:

• Cooling that’s weaker than normal, even though the system runs constantly
• Ice on the evaporator coil or the large refrigerant line
• Air that feels “cool-ish” but never truly cold
• A faint hissing sound near the outdoor or indoor unit

Why this becomes a professional diagnostic: Proper refrigerant diagnosis requires measuring pressures and temperatures, verifying airflow, and confirming the system is operating within manufacturer specs. Just “adding refrigerant” without repairing a leak usually means the problem comes right back—often during the hottest week of the year.

What this tells you: If you’ve ruled out airflow issues and the outdoor coil is clean, low refrigerant becomes more likely—and it’s time to call a licensed technician.

Watch for Electrical Start Problems (Capacitor/Contactor)

If the indoor fan runs but the outdoor unit won’t start—or it starts and stops, clicks, or hums—an electrical component may be failing. Two common parts involved are the capacitor (helps motors start/run) and the contactor (a switch that tells the outdoor unit to engage).

Signs you may notice:

• Outdoor unit makes a clicking sound but the fan doesn’t spin
• You hear humming, but nothing fully starts
• The system works sometimes, then quits during the hottest part of the day
• The breaker trips when the system tries to start

What you can safely do:

• Confirm the outdoor disconnect hasn’t been turned off.
• Make sure the thermostat is calling for cooling (Section 1).
• Look for obvious debris around the fan grille (without reaching inside).

Important safety note: Even with power off, capacitors can store a dangerous charge. This is not a DIY repair. If the outdoor unit isn’t starting normally, schedule an appointment so a technician can diagnose it safely.

What this tells you: Start issues are often fixable parts—not necessarily a full system replacement—but they need the right diagnosis quickly to prevent damage to larger components.

Check the Condensate Drain and Float Switch

Your AC removes humidity from indoor air. All that moisture becomes water in the drain pan and flows out through the condensate drain line. In humid weather, that line can grow algae and clog. When it backs up, many systems trigger a float switch that shuts the unit down to prevent water damage.

Signs of a drain issue:

• Water in the drain pan or around the indoor unit
• Musty odors near the air handler/closet
• The system shuts off unexpectedly, especially during long run times

What you can check safely:

• Look for standing water in the emergency pan (if you can see it).
• Find where the drain line exits the home and see if water is dripping while the AC is running (some setups make this easy to spot).

Simple prevention: If your system has a drain access opening, a small amount of distilled vinegar can help slow algae growth. (If you’re not sure where the access is, don’t guess—ask during your next maintenance visit.)

When this becomes a service call: If the unit keeps shutting down, you see water damage risk, or you can’t safely clear the line, get a technician involved. Water problems can damage ceilings, floors, and equipment if they’re ignored.

What this tells you: Drain issues can look like “AC isn’t cooling,” but the root problem is a safety shutoff. Clearing it quickly often gets you back online.

When to Call for AC Repair in San Antonio

If you’ve worked through the safe checks above and your system still isn’t cooling—or you suspect refrigerant or electrical problems—it’s time to bring in a professional. The goal is simple: get the right diagnosis quickly so you don’t risk bigger damage (especially to the compressor).

If you’re looking for AC repair San Antonio families rely on, 1st Call Plumbing Heating & Air is a full-service HVAC and plumbing company based in San Antonio with 35+ years serving the Greater San Antonio area, including New Braunfels, Boerne, Spring Branch, and nearby communities. They’re known for fast, courteous service with no trip charges, plus free estimates and financing options (with approved credit). They’re licensed by the Texas Department of Licensing and Regulation and the Texas State Board of Plumbing Examiners, and they’re highly rated with 1,900+ reviews and a 4.9-star average. You can learn more about our air conditioning service options here.

Call right away if:

• The outdoor unit won’t start, keeps clicking, or trips breakers
• You see ice repeatedly after changing the filter and thawing the system
• You suspect a refrigerant leak (hissing, weak cooling, recurring ice)
• You smell burning or see signs of electrical damage
• Water is backing up at the indoor unit

To speed up your service call, write down:

• How long the problem has been happening
• Whether the outdoor unit fan is spinning
• Whether you saw ice anywhere (and where)
• What you already tried (filter, thermostat settings, rinse, breaker reset)

Quick 5-step checklist

• Thermostat: Cool mode, fan on Auto, fresh batteries, setpoint lowered 3–5 degrees.
• Filter + airflow: Replace a dirty filter; confirm return grille and vents aren’t blocked.
• Ice: If frozen, turn cooling off and thaw fully before restarting.
• Outdoor unit: Clear debris/vegetation and rinse the coil gently.
• Know when to stop: Refrigerant and electrical issues need a licensed pro.

Frequently Asked Questions

How long should I wait after changing the filter?

If the system was simply starved for airflow, you may notice improvement within 30–60 minutes. If the coil was iced, you need to thaw it fully first—then test cooling again.

My vents are blowing air, but it isn’t cold. Is my compressor bad?

Not always. Weak cooling can come from airflow issues, a dirty outdoor coil, electrical start problems, or refrigerant issues. A compressor failure is possible, but it shouldn’t be the first assumption without proper testing.

Why does it cool at night but not during the afternoon?

That pattern often points to a system that’s struggling to reject heat during peak outdoor temperatures—commonly because of a dirty outdoor coil, restricted airflow, or low refrigerant. It can also happen with failing electrical parts that act up when they get hot.

Conclusion: Get the Cooling Back Safely

An AC that runs but doesn’t cool is usually caused by airflow problems, a dirty outdoor coil, a drain safety shutoff, or a mechanical issue like low refrigerant or a failed electrical component. A few basic checks can save time, but pushing past the safe DIY line can get expensive fast.

For fast, reliable AC repair San Antonio homeowners can trust, request a free service estimate from 1st Call Plumbing Heating & Air or call 210-446-0200. If you’d rather book online, you can schedule an appointment anytime. Emergency service is available, and they’re open seven days a week from 7:00 AM to 7:00 PM.