Why Is My AC Running But Not Cooling?

An air conditioner that runs but will not cool is nearly always an airflow problem or a refrigerant problem — not a dead system. The four causes that come up most are a clogged air filter, a frozen evaporator coil, a failed capacitor, and a refrigerant leak. Check the filter and the thermostat fan setting first. Those two cost nothing to rule out and they account for a real share of the calls that turn out to be nothing.

Why Is My AC Not Cooling? The Short Answer

If your air conditioner is running but not cooling, or the whole house is not getting cold enough, the cause is almost always one of five things: a clogged filter, a frozen evaporator coil, a dirty condenser, a failed capacitor, or low refrigerant from a leak. A central AC that suddenly stopped cooling the house usually points at the capacitor or the outdoor unit; an AC that is slowly not cooling enough usually points at airflow or a slow leak. The checks below tell the two apart in about five minutes.

One thing worth ruling out first: if the air at the vents is cold but the house will not cool, the equipment is working and the cool air is being lost on the way — usually to duct leaks in a hot attic. If the air at the vents is not cold at all, work through the list below.

The One Test That Tells You the Most

Before you touch anything, measure the split — the difference between the air going into your system and the air coming out of it.

1. Run the AC for 15 minutes.
2. Hold a thermometer at a return vent (the big one that sucks air in). Note the number.
3. Hold it at a supply vent in the same part of the house. Note that number.
4. Subtract.

A healthy system gives you 16 to 22 degrees colder at the supply vent — the same reading a technician takes on an annual tune-up. What you find tells a technician — and you — roughly where the fault sits:

Your split	What it usually means
16–22°F	The equipment is doing its job. If the house still will not cool, the problem is duct loss, insulation, or sizing — not the AC.
8–15°F	Low refrigerant, a dirty coil, or restricted airflow.
Under 5°F	The compressor is probably not running, or the charge is badly low.
Over 25°F	Airflow is choked — filter, blower, or a coil starting to ice.

On a very humid day the split narrows a little, because the system is spending energy pulling water out of the air instead of dropping temperature. That is normal and it is not a fault.

Check These Three Things Before You Call Anyone

1. Is the thermostat fan set to ON instead of AUTO?

This one is worth checking first because it costs nothing and it explains a lot of “my AC blows warm air” calls.

On AUTO, the blower only runs while the system is actively cooling. On ON, the blower runs constantly — including in the gaps between cooling cycles, when the coil is not cold. So you get cold air, then room-temperature air, then cold air again. It feels exactly like a failing system. Switch it to AUTO and give it an hour.

2. When did you last change the filter?

Pull it out and hold it up to a light. If you cannot see light through it, it is choking your system. A 1-inch filter in a Texas summer needs changing every 30 to 60 days, and closer to 30 if you have pets or anyone in the house has allergies. It is also the first item on any honest maintenance visit checklist.

A blocked filter does not just reduce airflow. It starves the evaporator coil of the warm air it needs to absorb heat from, the coil drops below freezing, and moisture on it turns to ice. Now you have a second problem stacked on the first.

3. Is the outdoor unit actually running?

Walk outside while the thermostat is calling for cooling. You want to see the fan on top spinning and hear the compressor humming. Three things you might find instead:

• Nothing at all — silent. Check the breaker panel, and check the outdoor disconnect box on the wall beside the unit.
• Humming but the fan is not turning. Classic failed capacitor. Turn the system off at the thermostat and stop — running it like this can cook the compressor.
• Running, but buried in grass clippings, leaves, or cottonwood seed. It cannot dump heat if it cannot breathe.

Airflow Causes

This is the biggest group by a wide margin. If your split reads high but the house is not cooling, look here first.

A frozen evaporator coil

The evaporator sits inside — in the attic, a closet, or the garage depending on the house. When it ices over, air can barely pass through it, so almost nothing cold reaches your rooms even though the system sounds like it is working hard.

Signs: weak air from the vents, visible frost on the copper line at the outdoor unit, water pooling near the indoor unit as it melts.

What to do: switch cooling off at the thermostat but set the fan to ON. Moving room-temperature air across the coil thaws it far faster than leaving everything off. Expect two to six hours. Change the filter while you wait.

Thawing it is not the fix, though — it is the symptom clearing. Something made it freeze, and if that is a refrigerant leak rather than a dirty filter, it will ice up again within a day or two.

A dirty condenser coil

The outdoor unit’s job is to release the heat your system pulled out of the house. The fins around the outside are the radiator. When they pack with cottonwood seed, grass clippings, or dryer lint, that heat has nowhere to go and the system just cycles without ever getting ahead.

Late spring across the Mid-Cities is the worst of it — cottonwood seed mats onto a condenser like felt, which is why a spring maintenance visit is timed for exactly then. You can see it from three feet away. Gentle rinsing with a garden hose from the inside out helps; a pressure washer will flatten the fins and make things permanently worse.

Leaking or crushed ductwork

This is the one people almost never guess, and in older houses it is one of the most common answers of all.

This is most common in the older housing across Haltom City and Watauga. If your ducts run through the attic — which they do in most homes built here between the 1950s and the 1990s — that attic sits at 130°F or more on a July afternoon. A separated joint or a flex run crushed under a stored box dumps the cold air you paid for straight into that attic. The equipment is fine. Your split at the coil would measure perfectly. The air just never arrives.

The tell: one or two rooms are noticeably worse than the rest, usually the ones furthest from the unit or directly upstairs.

Closed or blocked registers

Worth 60 seconds. Walk the house and check that supply vents are open and not sitting behind a sofa, a bed, or a stack of boxes. Closing vents in unused rooms is a popular idea that backfires — it raises pressure in the duct system, which increases leakage and can freeze the coil.

Electrical Causes

A failed capacitor

The single most common AC failure there is. The capacitor stores the jolt of energy the fan motor and compressor need to start turning. Texas heat kills them — they are the part most likely to give out in the middle of a July afternoon.

Signs: the outdoor unit hums but the fan does not spin. Or the fan spins but the compressor never kicks in, so air moves through the house at room temperature. Sometimes the fan starts if you nudge it with a stick through the grille — that is a capacitor, confirmed.

A capacitor tested with a meter during an annual tune-up is usually reading low months before it fails outright — which is the difference between a planned repair and a July emergency.

Safety, and this one matters: a capacitor holds a lethal electrical charge after the power is off, sometimes for a long time. It has to be discharged with the right tool before anyone touches the terminals. This is not a job to watch a video about. Every other cause on this page is either free to check or safe to inspect — this is the exception.

A failed contactor

The contactor is the switch that feeds power to the outdoor unit when the thermostat calls for cooling. Its contacts pit and burn over years of arcing. When it fails, the outdoor unit stays dead while the indoor blower carries on happily — so you get plenty of air, none of it cold.

A tripped breaker on the outdoor unit only

Most systems run on two circuits: one for the indoor air handler, one for the outdoor condenser. If only the outdoor breaker trips, the blower keeps running and blowing warm air, which is why people rarely think to check the panel. Reset it once. If it trips again, stop — a breaker that keeps tripping is reporting a real electrical fault, not being fussy.

Refrigerant Causes

Refrigerant is not fuel. It is not used up, and a system that was charged correctly should never need topping up. If you are low, you have a leak.

Signs: cooling that got gradually worse over weeks rather than failing overnight, ice on the copper line at the outdoor unit, a faint hissing or bubbling near the indoor unit, and longer and longer run times.

Anyone who offers to “just add a bit of gas” without finding the leak is selling you the same repair again next summer. Handling refrigerant also requires EPA Section 608 certification, so this is not a DIY item under any circumstances. If the system turns out to run the older refrigerant, the numbers change sharply — what R-22 refrigerant costs in 2026 explains why.

One thing worth knowing before you get a quote: if your system was installed before about 2010 it likely runs R-22 refrigerant, which has not been produced in the United States since 2020. Existing stock only, and the price reflects that. A leak repair on an R-22 system can pass $1,000 on refrigerant alone, which often changes the repair-or-replace conversation entirely.

What If Nothing Is Actually Broken?

Sometimes the honest answer is that the system is working and the weather is winning.

A residential air conditioner can only pull the indoor temperature about 20 degrees below whatever it is outside. At 104°F, a correctly functioning system holds the house somewhere around 78 to 84. If your thermostat is set to 72 and the room reads 79, nothing has failed — you are at the physical limit of the equipment.

Setting it to 68 does not cool the house faster. It just runs the compressor continuously against a target it can never reach, which raises the bill and wears the system out. We go through this in more detail on the homepage, along with what each repair on this page typically costs.

Which of These Can I Check Myself?

Do it yourself	Call someone
Thermostat set to AUTO, not ON	Anything involving a capacitor
Change or check the air filter	Refrigerant — legally requires certification
Clear debris around the outdoor unit	A breaker that trips more than once
Open and unblock supply vents	Contactor, control board, or wiring
Reset a tripped breaker — once	Duct repair in the attic
Thaw a frozen coil with fan-only	A coil that freezes again after thawing

Age matters here too. A fault that is worth repairing on a six-year-old system often is not on a fifteen-year-old one — how long an AC unit lasts in Texas walks through where the line sits, and a spring tune-up is how most of these faults get caught before they become a July emergency.

When Is This an Emergency?

Most no-cool calls can wait until morning. These cannot:

• Indoor temperature climbing past 85°F with an infant, an elderly person, or anyone with a heart or respiratory condition in the house
• A burning smell, or any smell of hot electrical insulation
• Water actively running from the indoor unit into a ceiling or wall
• A breaker that trips again immediately after being reset

If you are in one of those, here is what after-hours AC repair actually costs and how it works — including the trip charge, which you should be told on the phone before anyone is dispatched, not after the work is done.

Why This Happens More in Older Mid-Cities Homes

A lot of housing across Hurst, Bedford, Haltom City and North Richland Hills went up between the 1950s and the 1980s. Three things follow from that, and they change which cause on this page is most likely:

• Return air is often undersized. Houses of that era were built with smaller returns than a modern system wants, which pushes the whole system toward the airflow problems above.
• Ducts run through brutal attics. Original or near-original flex duct in a 130°F attic leaks, and the leaks get worse every year.
• Equipment installed before 2010 likely runs R-22. That changes the maths on any refrigerant repair.

It also means something practical: in an older house, a technician who checks the ductwork and the attic before recommending a new condenser is doing the job properly. One who quotes a replacement from the driveway is not. We go into the housing stock city by city — North Richland Hills splits into two eras, Grapevine spans more than a century of building, and Southlake runs the largest systems we see. Haltom City and Bedford are covered too.

Frequently Asked Questions

Why is my AC blowing cold air but the house is not getting cooler?

If the air at the vents is genuinely cold, the equipment is working and the cold air is being lost on the way — or the house is gaining heat faster than the system can remove it. Duct leaks in the attic are the usual answer. Undersized equipment and poor insulation are the next two.

How long should it take my AC to cool the house down?

Roughly 1 to 2 degrees per hour on a hot afternoon, more once the sun is off the house. A system that has been off all day in July can take several hours to catch up. That is normal. A system that runs all night and never reaches the setpoint is not.

Should I turn my AC off if it is running but not cooling?

Yes, in two cases: if the outdoor unit is humming without the fan turning, and if you can see ice anywhere on the system. Both get worse and more expensive the longer the system runs. Otherwise you can leave it running while you work through the checks above.

Can a dirty filter really stop an AC from cooling?

Yes, completely. A blocked filter starves the evaporator coil, the coil freezes solid, and a block of ice will not let air through. It is one of the cheapest possible causes and one of the most common.

Is it worth repairing an AC that is 15 years old?

It depends entirely on which part failed. A $300 capacitor on a 15-year-old system is usually money well spent. A $2,500 compressor on the same system usually is not. Multiply the age by the repair quote — if the result tops $5,000, replacement tends to be the better financial decision.

Why did my AC suddenly stop working?

A sudden, total stop usually means power or a failed start component — a tripped breaker, a blown capacitor, or a failed contactor in the outdoor unit. Check the breaker and the outdoor disconnect first. If the unit hums but the fan will not spin, that is a classic capacitor and worth turning off until it is fixed.

Why is my AC running but not getting cold enough?

Gradual loss of cooling — the AC runs constantly but never quite reaches the setpoint — is different from a sudden stop. It points at airflow or refrigerant: a dirty filter, a coil that needs cleaning, or a slow refrigerant leak that has been developing for weeks. Start with the filter, then measure the temperature split described above.

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

That pattern points at capacity rather than a fault. At night the outdoor temperature drops, the load falls, and the system catches up. During the day it is at its limit. Common causes are a system that is slightly undersized for the house, a dirty condenser coil, low refrigerant, or attic duct loss — all of which cost you nothing at 10pm and everything at 4pm.