It’s one of the most frustrating summer problems: you hear the air conditioner running, you feel a little airflow at the vents, but the house just keeps getting warmer. The system sounds “on,” the thermostat looks normal, and yet you’re sweating anyway. When that happens, the issue is usually something simple (like airflow) or something that’s quietly gotten worse over time (like a refrigerant leak).

This guide walks through the most common reasons an AC can run without cooling, how to tell what’s going on, what you can safely try yourself, and when it’s time to bring in a pro. While this article is published on synergiesprairies.ca, the troubleshooting steps apply broadly—especially in places with long cooling seasons where equipment runs hard and small issues show up fast.

One quick note before we dive in: if you smell burning, see smoke, hear loud metal-on-metal sounds, or notice the breaker tripping repeatedly, shut the system off and get help. Otherwise, let’s work through the likely culprits in a logical order.

Start with the simple checks that solve a surprising number of “no cooling” calls

Before assuming the worst, it helps to rule out the basics. Many “AC running but not cooling” situations come down to the thermostat setup, a clogged filter, or something blocking airflow. These are quick to check, and they can prevent bigger issues like frozen coils.

Think of your AC like a breathing system: it needs to pull warm air in, remove heat, and push cooled air back out. If any part of that airflow loop is restricted, the system can run nonstop and still fail to cool properly.

Thermostat mode, fan setting, and setpoint

It sounds obvious, but it’s worth confirming. Make sure the thermostat is set to Cool, not Heat or Auto (some smart thermostats can behave differently depending on schedules). Then verify the setpoint is at least a few degrees below the current room temperature.

Also check the fan setting. If the fan is set to On instead of Auto, the blower will run continuously—even when the outdoor unit isn’t actively cooling. That can make it feel like the AC is “running” without cooling, especially if the compressor is short-cycling or not coming on at all.

If you recently changed thermostat batteries or had a power outage, double-check the schedule and system type settings. A misconfigured thermostat (for example, set up for a heat pump when you have conventional AC) can cause confusing behavior.

Air filter condition and return-air blockage

A dirty air filter is one of the most common reasons cooling performance drops. When the filter is clogged, the system can’t move enough air across the evaporator coil. That reduces heat transfer and can even cause the coil to freeze, which makes cooling fall off a cliff.

Pull the filter and hold it up to a light. If you can’t see much light through it, replace it. In peak summer, some homes need a new filter every 30 days (or sooner if you have pets, construction dust, or wildfire smoke).

While you’re at it, make sure the return vents aren’t blocked by furniture, rugs, or heavy curtains. An AC can’t cool what it can’t pull in.

Supply vents, registers, and closed doors

Walk through the house and check supply vents. If several are closed, you can create pressure issues that reduce overall airflow and strain the blower. This is especially noticeable in homes with older ductwork or rooms that were added later.

Also consider interior doors. In some homes, closing doors can reduce return airflow from bedrooms, making those rooms feel stuffy and warm even if the rest of the house is okay. If you notice certain rooms not cooling, airflow balancing may be part of the fix.

If airflow feels weak everywhere, that points more strongly toward a filter, blower, duct issue, or a frozen coil—topics we’ll cover next.

When airflow is the real problem (even though the AC “sounds” normal)

Many people judge AC performance by sound: if they hear the indoor fan, they assume cooling is happening. But the indoor fan can run while the cooling process is failing. Airflow issues are sneaky because the system still “does something,” just not enough.

Airflow problems can also trigger secondary failures—like coil icing—which then causes even worse cooling. The earlier you catch airflow issues, the less likely you are to end up with a bigger repair.

Frozen evaporator coil (ice on the indoor unit)

If your AC is running but not cooling, a frozen evaporator coil is a top suspect—especially if airflow at the vents is weak. The evaporator coil is inside the indoor unit (often above the furnace or air handler). When it freezes, air can’t pass through properly, so the house stops cooling.

Signs of a frozen coil include: reduced airflow, a hissing or bubbling sound (sometimes), and visible frost on the copper lines near the indoor unit. You may also see water around the unit after it thaws.

What to do: turn the system off at the thermostat and set the fan to On to help thaw the coil (unless you suspect electrical issues). Thawing can take a few hours. Replace the filter, make sure vents are open, and then try cooling again. If it freezes again, the underlying cause could be low refrigerant, a blower problem, or a dirty coil—at that point, professional service is the smart move.

Dirty evaporator coil (less common to spot, very common in real life)

Even if the filter is changed regularly, the evaporator coil can still get dirty over time. Dust, pet hair, and fine debris can bypass the filter or enter during filter changes. A dirty coil reduces heat transfer, which means the system runs longer and cools less.

Unlike a dirty filter, you usually can’t diagnose a dirty coil at a glance unless you open panels (which may not be homeowner-friendly). If you’ve had repeated airflow issues, higher energy bills, or a musty smell when the AC runs, it’s worth having the coil inspected and cleaned.

Coil cleaning is not just “spray and go.” Done incorrectly, it can bend fins or cause drainage problems. A technician can also check the coil temperature and refrigerant pressures to confirm whether the coil is the main bottleneck.

Blower motor or capacitor issues

The indoor blower is responsible for moving air across the evaporator coil and through your ducts. If the blower motor is failing, or if its capacitor is weak, you might get some airflow but not enough to cool effectively.

Clues include: airflow that starts strong and then fades, a humming sound without much air movement, or a blower that runs intermittently. Some systems will throw an error code, but many won’t.

Because blower components involve electrical testing and sometimes high voltage, this is a good “call a pro” category. The fix might be as simple as a capacitor replacement, but you don’t want to guess on electrical parts.

Outdoor unit problems: the system runs, but the heat can’t leave your home

Your AC doesn’t “create cold” so much as it moves heat from inside to outside. The outdoor unit (condenser) is where that heat gets dumped. If the outdoor unit can’t do its job, indoor cooling suffers, even if the indoor fan is blowing like normal.

Outdoor issues are also common during heat waves because the condenser has to work harder right when dust, cottonwood fluff, and yard debris are most active.

Dirty condenser coil (outdoor coil clogged with debris)

If the outdoor coil is covered in dirt, grass clippings, or lint-like debris, it can’t release heat efficiently. That forces the compressor to run hotter and longer, and cooling capacity drops. In some cases, the system can shut down on high pressure or overheat protection.

Look for obvious debris on the fins around the outdoor unit. Make sure there’s at least 2 feet of clearance around it—trim shrubs and remove anything leaning against the coil. If you’re comfortable, you can gently rinse the coil from the outside with a garden hose (no pressure washer), but avoid bending fins.

If the coil is heavily impacted or the unit is still struggling after cleaning, a technician can do a deeper clean and verify the fan and refrigerant pressures are in the right range.

Condenser fan not spinning or spinning slowly

The outdoor fan pulls air through the condenser coil. If it’s not spinning, the system may still “run” in the sense that the thermostat is calling for cooling and the indoor fan is blowing—but the outdoor unit can’t reject heat. That often results in warm air at the vents.

Common causes include a failed capacitor, a worn fan motor, or debris jamming the blades. If you see the fan not moving while the unit is humming, turn the system off to avoid damaging the compressor.

This is another situation where professional diagnosis is best. A fan motor or capacitor replacement is usually straightforward for a tech, but letting the compressor overheat can turn a small repair into a major one.

Electrical disconnects, breakers, and contactors

Sometimes the indoor unit runs while the outdoor unit is completely off. That can happen if the outdoor disconnect is off, a breaker is tripped, or a contactor has failed. You’ll feel airflow inside but no cooling because the compressor isn’t running.

Check the electrical panel for a tripped breaker. If it trips again after resetting, stop resetting it—repeat trips can indicate a serious electrical issue. Also make sure the outdoor disconnect (usually a small box near the condenser) hasn’t been switched off.

Contactors and wiring issues require a technician. They’re common wear items, especially in hot climates where the system cycles frequently.

Refrigerant and heat transfer: when the AC can’t absorb enough indoor heat

Refrigerant is the working fluid that carries heat from inside to outside. Your AC is designed to operate with a precise refrigerant charge. If the charge is off, cooling drops, energy use rises, and the system can ice up or overheat.

Refrigerant issues are not DIY territory. Beyond the legal requirements in many areas, adding refrigerant without fixing the underlying problem (usually a leak) is like topping off a tire with a nail in it.

Low refrigerant from a leak

If your AC is low on refrigerant, it may run constantly and barely cool. You might also notice the larger copper line (suction line) getting unusually cold and sweating heavily, or the evaporator coil freezing.

Leaks can occur at service valves, brazed joints, or inside the coil itself. Some leaks are slow and show up as “it’s been getting worse for months,” while others are more sudden.

A proper repair involves finding the leak, fixing it, evacuating the system, and recharging to the manufacturer’s specifications. If you’re in a region where older refrigerants are being phased out, your technician may also discuss long-term options.

Incorrect refrigerant charge (too much or too little)

It’s not only low refrigerant that causes trouble. Overcharging can raise pressures and reduce efficiency, sometimes causing the system to trip on safety limits. Undercharging reduces cooling capacity and can cause icing.

If someone “topped off” refrigerant without measuring superheat/subcooling properly, the system might cool inconsistently or struggle on very hot days. This is why accurate charging matters—especially for high-efficiency systems that are less forgiving.

If your AC was recently serviced and now it’s performing worse, it’s reasonable to ask for a re-check of the charge and a full performance test.

Metering device problems (TXV or fixed orifice issues)

The metering device controls how much refrigerant enters the evaporator coil. If it’s restricted or failing, the system can run but deliver poor cooling, sometimes with symptoms that mimic low refrigerant.

TXVs (thermal expansion valves) can stick, and fixed orifices can clog. Diagnosing this requires pressure readings and temperature measurements at multiple points in the system.

Because metering device issues can be misdiagnosed, it’s worth working with a contractor who does thorough diagnostics rather than guessing and swapping parts.

Ductwork and distribution: cold air exists, but it’s not reaching you

Sometimes the AC is actually producing cold air, but the air isn’t making it into your living spaces effectively. Duct problems can waste a shocking amount of cooling—especially in attics, crawlspaces, or garages where ducts run through very hot areas.

If some rooms are cool and others are hot, or if the system cools at night but not during the day, distribution issues become more likely.

Leaky ducts and disconnected runs

Duct leaks can dump cooled air into an attic or crawlspace instead of your rooms. In extreme cases, a duct can become partially disconnected, and you’ll feel weak airflow at multiple vents even though the blower is working hard.

Clues include: dusty rooms, higher-than-normal energy bills, rooms that never match the thermostat, and noticeable temperature differences between floors. If you can safely access the attic, look for obvious disconnected ducts or crushed flexible ducting.

Professional duct sealing and repairs can dramatically improve comfort. It can also reduce runtime, which helps the equipment last longer.

Poor insulation around ducts in hot spaces

Even if ducts are connected, inadequate insulation can cause cooled air to warm up before it reaches the vents. This is common when duct insulation is old, torn, or compressed.

You might notice the air feels “cool-ish” near the air handler but lukewarm at the farthest vents. In very hot attics, the temperature gain can be significant.

Fixing duct insulation isn’t glamorous, but it’s one of those improvements that can make the whole house feel more even—especially during peak afternoon heat.

Air balancing and undersized ductwork

Some homes have airflow design issues: long duct runs, too many sharp bends, or undersized returns. The AC may be capable of cooling, but the air can’t move where it needs to go.

This often shows up after remodels or additions. A new room might be tied into existing ductwork without proper sizing, leaving it warmer than the rest of the home.

An HVAC pro can measure static pressure and airflow, then recommend solutions like adding a return, resizing ducts, or adjusting dampers. These changes can be the difference between “AC is always struggling” and “house feels normal.”

Drainage and humidity issues that make cooling feel weak

Cooling isn’t just about temperature; it’s also about humidity. If your AC isn’t removing moisture properly, the house can feel sticky and warmer than the thermostat reading suggests. Drainage problems can also trigger safety switches that shut down cooling while the fan keeps running.

These issues can be subtle because you may still get airflow, and the system may appear to be operating—just not delivering that crisp, dry comfort you expect.

Clogged condensate drain line

Your AC pulls moisture out of the air, and that water needs to drain away. If the condensate line clogs (often with algae or sludge), water can back up into the drain pan. Many systems have a float switch that shuts off the compressor to prevent overflow damage.

When that happens, the indoor fan may still run, making it seem like the AC is on—but cooling stops. You might notice water near the indoor unit or a musty smell.

Clearing a drain line can sometimes be a DIY job (wet/dry vacuum on the drain outlet), but if the clog keeps returning, a technician can flush the line, check the pan, and confirm the safety switch is working correctly.

Evaporator coil temperature issues affecting dehumidification

To remove humidity, the evaporator coil needs to get cold enough for moisture to condense. If airflow is too high, refrigerant charge is off, or the system is oversized, dehumidification can suffer even if the air temperature drops a bit.

This is why some homes feel clammy even when the thermostat says 74°F. Comfort depends on both temperature and relative humidity.

If humidity is a consistent issue, ask about blower settings, system sizing, and whether a whole-home dehumidifier makes sense for your climate and lifestyle.

Heat pumps and dual-purpose systems: cooling problems with a few extra twists

If you have a heat pump, the system cools in summer and heats in winter by reversing the refrigerant flow. That means some components—like the reversing valve and defrost control—play a role in cooling performance too.

Heat pumps are great for efficiency, but when something is off, symptoms can be confusing because the system may partially operate while still failing to cool properly.

Reversing valve issues

The reversing valve changes the direction of refrigerant flow. If it’s stuck or not energizing correctly, the system may not fully switch into cooling mode. You might get lukewarm air or inconsistent performance.

This problem requires professional diagnosis because it involves electrical signals and refrigerant circuit behavior. Sometimes the issue is the valve itself; other times it’s the control board or wiring.

If your heat pump cools fine some days and struggles on others, or if you notice odd temperature swings, it’s worth mentioning the possibility of a reversing valve or control issue when you call for service.

Defrost control or sensor problems (less common, but real)

Defrost mode is primarily a winter function, but a control problem can cause abnormal operation. If the system gets stuck in a mode that doesn’t match the season, comfort suffers.

Most homeowners won’t be able to diagnose this by sight. What you can do is pay attention to patterns: does cooling drop after certain cycles, or does the outdoor unit behave oddly?

Detailed symptom notes help your technician diagnose faster, which can reduce labor time and get you back to comfort sooner.

When upgrading is part of the fix

Sometimes the “fix” for repeated cooling issues is stepping back and looking at the whole system—especially if the equipment is older, uses phased-out refrigerant, or has a history of leaks and major repairs. In those cases, investing in an upgrade can be more cost-effective than chasing problems.

If you’re considering a heat pump for better year-round efficiency, it helps to talk through sizing, duct compatibility, and comfort goals. Many homeowners also like the idea of electrification and reducing reliance on gas.

For readers exploring this path, a service provider experienced with heat pump installation can explain what changes (if any) your home needs to get the best performance in both heating and cooling seasons.

What you can safely do right now vs. what should wait for a technician

When your home is hot, it’s tempting to try everything at once. A better approach is to do a few safe checks that won’t risk equipment damage, then stop and call for help if the problem persists.

Here’s a practical split between homeowner-friendly steps and tasks that are best left to professionals.

Safe homeowner steps (low risk, high value)

Replace the air filter, open supply vents, and ensure return vents are unblocked. Confirm thermostat settings (Cool mode, setpoint below room temp, fan on Auto). Check that the outdoor unit has power and isn’t buried in debris.

If you suspect a frozen coil, turn off cooling and run the fan to thaw. Keep an eye out for water during thawing, and don’t restart cooling until airflow is back to normal.

You can also rinse the outdoor coil gently with a hose if it’s visibly dirty—again, no pressure washing, and never poke objects into the fins.

Stop-and-call situations (to prevent bigger damage)

If the outdoor fan isn’t spinning, the breaker keeps tripping, you hear loud buzzing or grinding, or you see ice repeatedly forming, it’s time to stop. Continuing to run the system can damage the compressor, which is one of the most expensive components.

Likewise, if you suspect a refrigerant leak (hissing, oily residue on lines, repeated icing), don’t run it “to see if it gets better.” It generally won’t, and it can get worse quickly during extreme heat.

Water around the indoor unit can also signal a drain issue or coil freeze. Either way, it’s worth getting checked before moisture causes secondary damage.

What a good service visit should include

A thorough technician won’t just “top off refrigerant” and leave. They should check airflow, temperature split (return vs. supply), coil condition, refrigerant pressures, electrical components (capacitors, contactors), and overall system operation.

They may also look at duct static pressure and confirm the outdoor unit is rejecting heat properly. If your problem is intermittent, sharing the timeline—when it happens, outdoor temperatures, and any recent changes—can help a lot.

If you’re comparing providers, look for clear communication, transparent pricing, and a diagnostic process that makes sense rather than guesswork.

When it’s time to bring in local help (and how to choose the right team)

Some issues—like a clogged filter—are easy wins. Others require tools, training, and experience. If you’ve gone through the basic checks and the system still isn’t cooling, calling a professional is the fastest route back to comfort (and often cheaper than letting the problem snowball).

Choosing the right contractor matters because “AC running but not cooling” can have multiple overlapping causes. A good team will test, verify, and explain what they found in plain language.

What to look for in an HVAC contractor

Look for licensing and insurance, strong local reviews, and technicians who are willing to show measurements (like temperature split and pressures) rather than relying on vague statements. Ask whether they check airflow and duct issues, not just refrigerant.

It also helps to find a contractor who offers maintenance plans or seasonal tune-ups. Preventive service—cleaning coils, checking capacitors, clearing drains—can prevent the exact scenario you’re dealing with now.

If you’re in the North Bay area and want a starting point, many homeowners search for Santa Rosa HVAC contractors who can handle both diagnostics and longer-term improvements like duct repairs, indoor air quality upgrades, and system replacements when needed.

Using maps and reviews to verify you’re calling a real local business

When you’re hot and stressed, it’s easy to click the first ad you see. Taking an extra minute to verify a business is truly local—and has consistent reviews—can save you from surprise fees or poor workmanship.

Checking a company’s listing, hours, and review patterns can help you feel more confident. It’s also a good way to confirm the service area and see whether the business is established in your community.

For example, you can view HVAC contractors Santa Rosa, CA on Google Maps to see location details and feedback in one place while you compare options.

How to describe the problem so you get faster answers

When you call, share a few specific observations: Is the outdoor unit running? Is the outdoor fan spinning? Is airflow weak or normal? Do you see ice on the refrigerant line? Did the issue start suddenly or gradually?

Also mention any recent changes: thermostat replacement, power outage, home renovation, filter type change (like switching to a very high MERV filter), or landscaping work near the condenser.

The more precise you are, the more likely the technician arrives prepared—sometimes even with the most likely parts on the truck.

Keeping your AC from “running but not cooling” next season

Once you’ve fixed the immediate issue, it’s worth putting a few habits in place to reduce the odds of a repeat. Most cooling failures aren’t random—they’re the result of gradual restrictions, wear, and missed maintenance.

A little proactive care can improve comfort, lower energy bills, and reduce emergency calls during the hottest week of the year.

Filter strategy that matches your home (not just what the packaging says)

Choose a filter rating that your system can handle. Very restrictive filters can reduce airflow if your ductwork or blower isn’t designed for them. If you want better filtration for allergies or smoke, it may be better to upgrade the filtration system rather than using an ultra-high-MERV 1-inch filter.

Set a reminder to check the filter monthly during heavy use. Even if it doesn’t need replacement every month, checking is quick and keeps you aware of how fast it loads up in your specific home.

If you have pets or do a lot of cooking, you may find your ideal replacement schedule is shorter than average.

Outdoor unit care that actually helps

Keep the area around the condenser clear. Don’t store items against it, and avoid planting shrubs too close. If you mow the lawn, try not to blast grass clippings directly into the fins.

During cottonwood season or dusty periods, a gentle rinse can help. Just be careful: fins are delicate, and bending them reduces airflow through the coil.

If you notice the unit is noisier than usual or vibrating, address it early—small mechanical issues can lead to bigger failures under summer load.

Seasonal maintenance and performance checks

A spring tune-up typically includes cleaning, electrical checks, condensate drain inspection, and a basic performance test. It’s not just about preventing breakdowns—it’s about ensuring the system is operating efficiently before the heat arrives.

If your home has comfort trouble spots, ask about airflow measurements and duct evaluation. Fixing distribution issues can make the whole system feel “stronger” without changing the equipment.

And if your system is older, maintenance visits are a good time to plan ahead—so you can replace on your schedule rather than during an emergency.

If your AC is running but not cooling, the good news is that many causes are fixable—often quickly—once you pinpoint whether the problem is airflow, outdoor heat rejection, refrigerant performance, or distribution. Start with the easy checks, protect the equipment by shutting it down when something seems off, and bring in a qualified technician when the symptoms point to electrical or refrigerant issues.