AC Blowing Warm Air? Here's What's Causing It and What It Costs to Fix

An AC blowing warm air is one of the most common service calls of the summer. The system is running — you can hear it, the thermostat says it's on — but the air coming out of the vents is somewhere between "room temperature" and "disappointingly warm." The outdoor unit may or may not be doing anything.

There are six main causes. Two of them you can fix yourself in under five minutes. Two require a technician but are relatively inexpensive. Two are serious and expensive. Here's how to tell which you're dealing with.

Check First: The Thermostat

Before anything else, check the thermostat settings. This is not a joke — it's the cause of more "AC blowing warm air" calls than most homeowners expect.

Confirm:

• The system is set to COOL, not HEAT or FAN ONLY
• The set temperature is lower than the current room temperature by at least 2 to 3 degrees
• The fan is set to AUTO, not ON

That last point matters. When the fan is set to ON, it runs continuously — even when the system isn't in a cooling cycle. The air it moves at those times is unconditioned room-temperature air, which feels warm compared to what you expect from AC.

If the thermostat checks out, move to the filter.

Cause 1: Dirty Air Filter

DIY fix. Cost: $5 to $25.

A clogged filter restricts airflow across the evaporator coil. Less airflow means the coil can't absorb heat from the air effectively, which reduces the system's cooling output. In some cases it causes the coil to freeze (more on that below), which makes things worse.

Pull out the filter. Hold it up to light. If you can't see light through it, it's restricting airflow and needs to be replaced now.

This is the most common cause of reduced cooling. It's also the cheapest fix. A $10 filter and two minutes of your time resolve it. If the system starts cooling properly within 15 to 20 minutes of the filter replacement, that was your problem.

Cause 2: Frozen Evaporator Coil

DIY fix (usually). Cost: $0 to $25 for the fix itself, plus filter if needed.

When airflow drops — from a dirty filter, blocked vents, or a refrigerant issue — the evaporator coil can drop below freezing. Ice builds up on the coil. A frozen coil blocks airflow almost completely, turning your AC into an expensive room-temperature fan.

Signs of a frozen coil:

• Ice or frost on the refrigerant lines running into the air handler (the thin copper pipes)
• Reduced or no airflow from the vents even though the system is running
• Water dripping around the air handler as ice melts during off cycles

What to do:

Switch the thermostat to FAN ONLY — not OFF, and not COOL. This runs the blower without activating the refrigerant cycle, which thaws the coil faster than simply turning the system off. Give it 2 to 4 hours.

While it's thawing, check and replace the air filter if needed, and verify all supply and return vents are open and unobstructed.

Once thawed, switch back to COOL and give the system 15 to 20 minutes to see if it's cooling normally.

If the coil freezes again after you've addressed the airflow, the cause is almost certainly low refrigerant — which means there's a leak. That's Cause 4 below.

Cause 3: Wrong Thermostat Settings (Fan ON vs. AUTO)

Already covered above, but worth reinforcing. If your AC was working fine yesterday and today the air feels lukewarm, check the fan setting before assuming anything is broken. The FAN ON setting is the most common accidental trigger for this complaint, especially after someone has been adjusting the thermostat.

Cause 4: Refrigerant Leak

Call a technician. Cost: $250 to $1,600.

Refrigerant is the substance that makes air conditioning possible — it cycles between liquid and gas states to absorb heat from inside your home and reject it outside. If your system is low on refrigerant, it can't absorb heat effectively, and the result is warm or barely cool air.

Refrigerant doesn't get "used up." If your system is low, it's leaking. The right repair is to find the leak, fix it, then recharge the system to the correct level. A company that just adds refrigerant without finding and fixing the leak is doing you a disservice — you'll have the same problem within a season, and you'll have paid twice.

Signs pointing to a refrigerant leak:

• Ice on the refrigerant lines or outdoor unit (despite clean filter and open vents)
• The system runs continuously but can't get the house below 78 to 80°F
• Hissing or bubbling sounds near the indoor air handler
• Electric bills noticeably higher with no change in usage
• A sweet, ether-like chemical smell near the indoor unit

Repair costs depend on where the leak is and how bad it is:

• Minor leak, accessible location: $250 to $500 (leak repair + refrigerant recharge)
• Moderate leak, more involved detection: $500 to $1,000
• Evaporator coil replacement (when coil is leaking and can't be repaired): $1,000 to $2,500

R-410A refrigerant — used in most systems installed before 2025 — is being phased out under the AIM Act, and prices have risen significantly. Current contractor pricing runs $30 to $75 per pound, and a full recharge on a 3-ton system requires 6 to 9 pounds.

Do not keep running the system with a known refrigerant leak. Low refrigerant causes the compressor to run hot and work harder than it should, which accelerates compressor failure. A $500 refrigerant leak repair becomes a $2,000 problem if you ignore it long enough.

Turn the system off and call a technician.

Cause 5: Condenser Unit Problems

Some DIY, some require a technician. Cost: $0 to $400.

The outdoor condenser unit — the large unit with the fan on top — is responsible for rejecting heat from the refrigerant into the outdoor air. If the condenser isn't doing its job, the system can run but not cool.

Check these yourself:

Is the outdoor unit running at all? When the thermostat calls for cooling, the condenser fan should be spinning. If the unit is silent and the fan isn't moving, the unit may have tripped an internal high-pressure or overheat cutout switch.

Is the unit buried in vegetation or debris? The condenser needs clear airflow on all sides to reject heat effectively. Shrubs, tall grass, or debris within 18 inches of the unit restrict that airflow. Clear everything within 2 feet. Rinse the condenser coil fins with a garden hose on a gentle setting — spray from the top down to clear dirt and cottonwood without pushing debris deeper.

For the technician:

If the condenser fan isn't spinning but you can hear the unit humming, the run capacitor has likely failed. This is a $150 to $400 repair — common, and usually completed in under 30 minutes. Don't try to diagnose or replace a capacitor yourself; they store significant electrical charge that can cause serious injury even when the power is off.

If the outdoor unit makes a grinding or screeching noise, the condenser fan motor may be failing. Motor replacement runs $300 to $600.

Cause 6: Compressor Failure

Call a technician. Cost: $1,500 to $3,500 for repair — replacement often more practical.

The compressor is the heart of the refrigerant circuit. It circulates the refrigerant under pressure between the evaporator and condenser. When it fails, the refrigerant stops moving and the system produces no cooling at all — not reduced cooling, but zero.

Signs the compressor has failed:

• The outdoor unit hums but the system produces no cooling whatsoever
• The compressor makes a loud clicking or grinding noise before shutting down
• The circuit breaker for the outdoor unit keeps tripping
• The system is over 10 to 15 years old and multiple components have already been replaced

Compressor replacement runs $1,500 to $3,500 for parts and labor. On a system under 10 years old still within its factory warranty period, this may make sense. On a 12- or 15-year-old system, it often doesn't. A new compressor on an aging system still leaves you with an aging system — the condenser coil, refrigerant lines, and evaporator coil are all at the same age and wear level.

Many technicians won't warranty compressor replacements on systems over 12 years old for exactly this reason.

If the compressor has failed on an older system, get a quote for system replacement alongside the compressor repair quote. Full central AC replacement (outdoor unit, coil, refrigerant, labor) runs $3,500 to $7,000 for most residential systems in 2026. The math often favors replacement.

Summary: What It Costs to Fix

When to Stop DIYing and Call

If you've replaced the filter, confirmed the thermostat settings, thawed a frozen coil (and addressed what caused it), and the system still isn't cooling — you're past the DIY phase. The remaining causes require refrigerant certification, electrical diagnostic equipment, or compressor expertise.

Call a technician if:

• The system cools but can't get the house below 80°F despite running continuously
• You see ice on the refrigerant lines after you've replaced the filter
• You hear a hissing, bubbling, or grinding sound from the indoor or outdoor unit
• The outdoor unit isn't running despite the thermostat calling for cooling
• The breaker for the outdoor unit keeps tripping

If you need a licensed HVAC technician, post a service request on HVACJobs.IO. Describe what the system is doing, what you've already checked, and your system's approximate age — that information helps the tech come prepared and reduces your diagnostic time.

Frequently Asked Questions

Why is my AC running but blowing warm air?
The most common causes are a dirty air filter restricting airflow, a frozen evaporator coil, a refrigerant leak, or a failed run capacitor on the outdoor unit. Start with the filter — replace it and give the system 15 to 20 minutes to see if cooling improves. If the coil is frozen, switch to FAN ONLY for 2 to 4 hours to thaw it, then restart.

Can I fix an AC blowing warm air myself?
Some causes are DIY-accessible: thermostat settings, filter replacement, and thawing a frozen coil are all things you can handle. Anything involving the refrigerant circuit, capacitors, or the compressor requires a licensed technician with EPA 608 certification.

How much does it cost to fix an AC blowing warm air?
Depends entirely on the cause. A filter fix costs $5 to $25. A refrigerant leak repair runs $250 to $1,600. A capacitor replacement is $150 to $400. Compressor failure — worst case — runs $1,500 to $3,500. A diagnostic visit ($75 to $200) tells you which category you're in.

Why does my AC blow warm air only sometimes?
Intermittent warm air often points to a refrigerant leak (which gets worse as the system runs and refrigerant levels drop), or a failing run capacitor that works sometimes and fails under sustained load. Both require a technician to diagnose. Note the pattern: does the problem happen after the system has been running for 30 to 60 minutes? That suggests the capacitor is failing under thermal load.

Why is my AC blowing warm air at night?
If the problem appears at night but not during the day, check whether the thermostat is set to a schedule that might be cutting the cooling cycle. Also check whether someone has set the fan to ON — overnight, when the temperature difference between indoors and outdoors narrows, the fan-only mode feels noticeably warm.

How do I know if my AC is low on refrigerant?
Signs include: the system runs continuously but can't cool the house below 78 to 80°F, ice forming on the refrigerant lines despite a clean filter, a hissing or bubbling sound near the indoor unit, and higher-than-normal electric bills. You cannot check refrigerant levels yourself — it requires gauges and EPA 608 certification to handle refrigerants legally.