Your air conditioner is humming and air is moving through the vents, yet the house keeps getting warmer. Maybe the air feels only slightly cool, or the thermostat creeps up a degree every hour on a hot afternoon. It is frustrating because the system looks like it is working.
This guide walks you through the likely causes in the order a technician would check them, starting with free five-minute fixes and ending with the problems that need a licensed pro. You will learn how to tell an airflow problem from a refrigerant or electrical fault, what repairs typically cost, and how to prevent a repeat.
Quick answer: Confirm the thermostat is set to COOL with the setpoint below room temperature, replace a dirty air filter, and make sure the outdoor unit is running with a clean coil. If the outdoor unit is silent, check its breaker and the disconnect box beside it. If you see ice on the copper lines, switch the system off to thaw. A coil that keeps freezing, a failed capacitor or a refrigerant leak needs an HVAC technician.
First, confirm what “not cooling” really means
Hold a thermometer in a supply vent near the indoor unit and another at the return grille, let the system run for about 15 minutes, and compare. Many technicians treat a drop of roughly 15 to 20°F as a loose rule of thumb, although the Air Conditioning Contractors of America notes that the right number depends on humidity, airflow and the equipment.
If the vent air is clearly cold but the house will not cool down, suspect airflow, ducts or a heat wave beyond what the system was sized for. If the air is barely cooler than the room, the problem is usually at the outdoor unit, the refrigerant circuit or the coil. Walk outside and listen: a healthy condenser has a spinning fan on top and the steady hum of a running compressor.
Quick troubleshooting table
| What you notice | Most likely cause | DIY or pro? |
|---|---|---|
| Indoor fan runs, outdoor unit silent | Tripped breaker, pulled disconnect, failed capacitor or contactor | Check power yourself; pro for parts |
| Outdoor unit hums, fan does not spin | Weak capacitor or failed fan motor | Pro |
| Weak airflow from every vent | Dirty filter, frozen coil, blower problem | DIY filter; pro if it continues |
| Ice on the copper line or indoor coil | Restricted airflow or low refrigerant | DIY thaw; pro if it returns |
| Water around the indoor unit, cooling stops | Clogged condensate line tripping the float switch | DIY flush |
| Air slightly cool, system runs nonstop | Dirty condenser coil or refrigerant leak | DIY rinse; pro for leaks |
| Some rooms hot, others fine | Closed registers, blocked returns, leaky ducts | DIY check; pro for ducts |
Causes you can check and fix yourself
1. Thermostat settings or dead batteries
Make sure the mode is COOL, not HEAT or FAN only, and that the setpoint is a few degrees below the current room temperature. If the fan is set to ON instead of AUTO, the blower keeps running while the compressor rests between cycles, so the vents can feel warm and humid at times.
Replace the batteries if the display is faint, and look for a schedule, vacation hold or energy-saving mode overriding your setting. A completely dead screen with fresh batteries can mean the thermostat has lost low-voltage power, often because a safety switch at the indoor unit has tripped.
2. A dirty air filter
A clogged filter is the most common cause of poor cooling. It chokes airflow across the indoor coil, so less heat is pulled from your home, and in bad cases the coil freezes. ENERGY STAR recommends checking the filter every month and warns that airflow problems can reduce a system’s efficiency by up to 15 percent.
Hold the filter up to a light, and if you cannot see light through it, replace it. Central air and furnaces usually share one filter, so our guide on how to change a furnace filter shows how to find the right size and point the airflow arrow correctly.
3. A tripped breaker or outdoor disconnect
Most central systems have one breaker for the indoor air handler or furnace and a separate 240-volt double breaker for the outdoor condenser. If the indoor blower runs but the outdoor unit is silent, look for a tripped breaker labeled AC or condenser. Reset it once by pushing the handle firmly to OFF, then back to ON.
Next, check the disconnect box on the wall beside the outdoor unit and make sure its pull-out block or lever is fully in the ON position. If the breaker trips again within minutes, leave it off, because repeated tripping points to a shorted compressor, motor or wiring fault. Our guide to a circuit breaker that keeps tripping explains why you should never keep resetting it.
Safety note: Never remove the condenser’s electrical access panel while power is on. The capacitor inside can hold a dangerous charge even after the power is shut off, so leave anything beyond resetting the breaker or reseating the disconnect to a technician.
4. A dirty condenser coil or crowded outdoor unit
The outdoor unit releases the heat removed from your house. When its fins are matted with cottonwood fluff, grass clippings or pet hair, or shrubs crowd the cabinet, it cannot shed heat fast enough and cooling capacity drops. Keep foliage and debris at least 2 feet (0.6 m) away from all sides of the condenser.
Switch off both the breaker and the disconnect, then clear debris from the top and base. Rinse the coil with a garden hose on a gentle shower setting, working from the top down. Never use a pressure washer, which flattens the thin aluminum fins; for stubborn grime, use a foaming coil cleaner made for condensers and follow the label.
5. A frozen evaporator coil
Frost or ice on the larger insulated copper pipe, or on the indoor coil itself, means the coil has frozen. Ice blocks airflow, so cooling drops off sharply while the system keeps running. The usual culprits are a dirty filter, closed vents, a weak blower or low refrigerant.
Set the thermostat to OFF and the fan to ON so the blower helps melt the ice, and give it several hours with towels around the indoor unit. Replace the filter before restarting. If it freezes again within a day or two, see our guide to an AC unit freezing up, because the cause is probably refrigerant or blower related.
6. A clogged condensate drain line
Your AC pulls moisture from the air, and that water runs through a PVC condensate line that algae and sludge can block. Many systems have a float switch that stops cooling when the drain pan fills, so a clogged line can look like an AC that runs its fan but never cools.
Look for standing water in the pan under the indoor coil. With the power off, hold a wet/dry vacuum against the outdoor end of the line for a minute or two to pull out the clog, then pour about a cup of distilled white vinegar into the access port near the indoor unit to slow regrowth.
Problems that need a licensed HVAC technician
If the air is still warm after the checks above, the cause is most likely inside the sealed refrigerant system or the electrical components. These repairs need gauges, meters and, for refrigerant, federal certification.
7. A refrigerant leak
Air conditioners do not use up refrigerant, so a low charge means a leak, often at a coil, valve or brazed joint. Signs include lukewarm air, a coil that keeps freezing, a hissing sound near the lines and rising electric bills. Under the EPA Section 608 program, refrigerant work is done by certified technicians, and intentionally venting refrigerant is prohibited.
Be wary of a contractor who only tops off the system every year without finding the leak. A proper repair means locating and fixing the leak, then recharging to the manufacturer’s specification. Older systems that use R-22 are costly to recharge because that refrigerant is no longer produced, which often tips the decision toward replacement.
8. A failed capacitor or contactor
The run capacitor gives the compressor and outdoor fan motor the electrical boost they need to start and keep running. When it weakens, you may hear the outdoor unit hum while the fan sits still. The contactor is a relay that switches the outdoor unit on when the thermostat calls for cooling, and a burned or stuck one can stop the condenser from starting or keep it running nonstop.
Both parts sit behind the condenser’s electrical panel with 240-volt power and stored charge. A technician can test them with a meter and confirm that the fan motor and compressor are healthy before replacing anything.
9. Blower, compressor or duct problems
Weak airflow from every vent, even with a new filter, can mean a failing blower motor or a dirty blower wheel. If only some rooms are hot, make sure registers are open and returns are not blocked by furniture, then suspect disconnected or leaky ducts in the attic or crawlspace. A failing compressor is the costliest cause: the outdoor fan runs but the air never gets cold, or the breaker trips again and again.
Tools and materials
- Two thermometers or an infrared thermometer
- Replacement air filter in the correct size
- Garden hose with a gentle spray nozzle
- Foaming condenser coil cleaner (optional)
- Wet/dry shop vacuum
- Distilled white vinegar and a small funnel
- Flashlight, towels and fresh thermostat batteries
How to prevent it next summer
Most of these problems can be prevented with a few seasonal habits. Do them in spring, before the first heat wave, when HVAC companies are easier to book.
- Check the filter monthly during the cooling season, as ENERGY STAR recommends, and replace it when it looks dirty.
- Rinse the condenser coil each spring and keep plants trimmed 2 feet back.
- Flush the condensate line with vinegar at the start of the season.
- Book a professional tune-up so a technician can test the capacitor, check the refrigerant charge and clean the indoor coil.
- Clear fallen leaves from around the outdoor unit in autumn, along with the other jobs on our fall home maintenance checklist.
When to call a pro
Call an HVAC technician if any of these apply:
- The breaker trips again after a single reset.
- The outdoor unit hums, buzzes or clicks but the fan does not spin.
- The coil freezes again after a new filter and a full thaw.
- You hear hissing, see oily residue on the refrigerant lines or smell burning.
- The vent air is barely cooler than the room with a clean filter and clean condenser.
Prices vary by region and system. This Old House puts most common AC repairs at about $150 to $650, and lists these typical ranges for specific jobs:
| Repair | Typical cost range |
|---|---|
| Diagnostic fee | $75 to $150 |
| Drain line cleaning | $80 to $250 |
| Contactor repair or replacement | $100 to $450 |
| Capacitor replacement | $150 to $400 |
| Refrigerant leak repair | $200 to $1,500 or more |
| Outdoor fan motor | $300 to $800 |
| Evaporator coil | $700 to $2,500 or more |
| Compressor | $1,000 to $3,000 or more |
If an older system needs a compressor or coil, or still runs on R-22, ask for a replacement quote too so you can compare both options.
Frequently asked questions
Why is my AC running but the house is not getting cooler?
The most common reasons are a dirty filter, a dirty or blocked outdoor coil, a frozen indoor coil or an outdoor unit that has lost power. If the vent air is cold but rooms stay warm, look at closed registers, blocked returns and leaky ducts.
Should I turn off my AC if it is not cooling?
Yes, if you see ice, smell burning or hear the outdoor unit humming without its fan spinning. Running it in those conditions can overheat the compressor, the most expensive part to replace.
How do I reset my air conditioner?
Set the thermostat to OFF, switch off the AC breakers for a few minutes, then turn them back on and set the thermostat to COOL. Many systems wait a few minutes before the compressor restarts, so be patient. A reset only helps after a power blip, not a dirty coil or failed part.
Can a dirty filter really stop an AC from cooling?
Yes. A badly clogged filter can cut airflow enough for the indoor coil to freeze solid, and once it ices over, very little cool air gets through. A new filter plus a full thaw fixes many no-cooling calls.
Is it normal for an AC to fall behind during a heat wave?
Some lag is normal, because systems are sized for typical local summer temperatures rather than record highs. If the vent air is still cold, close blinds on sunny windows and avoid using the oven during the hottest hours. If the vent air is barely cool, something else is wrong.
The bottom line
When your AC runs but does not cool, work from simple to serious: thermostat, filter, power to the outdoor unit, a clean condenser, ice on the lines and the condensate drain. Those checks solve many problems for the cost of a filter. If the outdoor unit hums without spinning, the coil keeps freezing or the air stays warm with everything clean, turn the system off and call a licensed technician before a small repair becomes a compressor replacement.
