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Why is my AC not cooling?
Quick answer
If your AC runs but blows warm or weak air, the problem is usually airflow, a dirty coil, or low refrigerant rather than a completely dead system. Start with the thermostat, filter, and outdoor unit before you assume the worst. Check one simple item at a time, keep hands off refrigerant lines and wiring, and stop if ice, burning smells, or repeated breaker trips appear.
Causes, most likely first
Clogged air filter
Beginner. A packed filter strangles airflow, so the system runs longer, cools less, and can eventually freeze the indoor coil. Pull the filter out and hold it to a light. If light barely passes through, replace it with the same size and airflow direction, then give the system an hour to recover.
Thermostat set or reading wrong
Beginner. A thermostat set to fan-only, heat, or a temperature above the room reading will never start a cooling cycle, and direct sun on the unit can fool its sensor. Set it to cool, lower the target five degrees, and wait ten minutes. Listen for the outdoor unit to start and feel for cool air at a vent.
Outdoor unit blocked or filthy
Beginner. The outdoor condenser has to dump heat outside, and shrubs, grass clippings, cottonwood fluff, or a fence panel tight against it trap that heat. Shut power off at the thermostat, clear plants back about two feet, and gently rinse the coil fins from outside with a garden hose. Never use a pressure washer.
Low refrigerant from a leak
Pro only. Refrigerant does not get used up. If the level is low, there is a leak, and the system will blow cool at first, then gradually warm, sometimes icing the indoor coil. You may notice hissing near a line or oily residue at a fitting. Only a licensed technician can legally recover, repair, and recharge refrigerant.
Failing capacitor or condenser fan
Pro only. When the outdoor fan will not spin, spins slowly, or hums and quits, heat cannot leave the system and cooling stops fast. A weak capacitor is a common reason, especially after a heat wave. Do not reach into the unit or try to start the fan by hand. Capacitors store a dangerous electrical charge.
Leaky or disconnected ducts
Intermediate. Cooled air that escapes into an attic or crawlspace never reaches your rooms, so one area stays warm while the unit runs nonstop. Check accessible duct joints for gaps, crushed flex duct, or fallen sections. Seal obvious joints with mastic or foil tape made for ducts, not cloth duct tape.
When to call a pro
- Ice on the indoor coil or refrigerant lines that returns after thawing
- Burning, electrical, or musty chemical smells from vents or the unit
- Breaker trips more than once, or the outdoor unit buzzes but never starts
- Hissing sounds, oily spots on lines, or a suspected refrigerant leak
- No cooling after a filter change and outdoor cleaning
- Any work involving wiring, capacitors, motors, or refrigerant
What the repair typically costs
Directional national ranges for US homeowners. Your city and system age will move these.
| Repair | Typical cost |
|---|---|
| Filter and basic homeowner cleaning | $10 to $40 in parts |
| Thermostat replacement | $150 to $350 |
| Capacitor replacement by a technician | $150 to $400 |
| Condenser fan motor replacement | $400 to $900 |
| Refrigerant leak search, repair and recharge | $600 to $2,000 and up |
| Blower motor replacement | $500 to $1,200 |
| Full system replacement, at end of life | See our HVAC replacement cost guide |
Questions homeowners ask
How long should I wait after changing the filter?
Give it one to two hours of normal running. Cooling recovers gradually, and a coil that started to freeze may need the fan running with cooling off for a few hours to thaw fully.
Is it safe to run an AC that is not cooling?
For a short test, yes. Running it all day wastes power and can damage the compressor, especially if airflow is blocked or refrigerant is low. Switch it off if you see ice or hear grinding.
Why is one room warm when the rest are fine?
That usually points to a duct problem, a closed or blocked register, or sun load on that room rather than a failing system. Check the register and the accessible duct run to that room first.