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AC not cooling: causes, fixes and what it costs
The blower is running, you can hear the outdoor unit, and the air from the vents is not cold. That combination narrows the fault list considerably, and most of it is diagnosable in one visit.
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Call (855) 321-3194Is this an emergency or can it wait?
It can usually wait a day, unless a heat advisory is in effect or someone in the home is vulnerable to heat. Shut the system off so it stops running against the fault, then call (855) 321-3194 and we connect you with a local provider.
What you are seeing
Cooling side symptoms
- Supply air is room temperature or warmer
- The system runs continuously without reaching setpoint
- Ice visible on the indoor coil or the line set
- The outdoor fan runs but the compressor does not
Airflow side symptoms
- Vents move very little air
- Some rooms cool and others do not
- A whistling note at the return grille
- The filter is grey and loaded
Do and do not
| Do | Do not |
|---|---|
| Set the thermostat to off so the system stops cycling | Keep lowering the setpoint hoping it catches up |
| Replace a visibly loaded filter | Run the system with no filter at all |
| Clear leaves and grit from the outdoor coil | Pressure wash the coil fins |
| Let ice melt fully before running it again | Chip or scrape ice off the coil |
| Note what changed and when, for the technician | Reset a tripping breaker repeatedly |
What causes it, ranked by how often
- Restricted airflow. A loaded filter or a dirty evaporator coil. The system cannot absorb heat, the coil runs too cold and eventually ices. Most common cause by a wide margin, and the least costly to put right.
- Low refrigerant charge from a leak. A sealed system does not consume refrigerant, so low charge means a leak. Capacity drops gradually, which is why people describe the system as getting weaker across a season.
- Failed capacitor. The compressor or fan motor will not start. You hear a hum rather than a start. Cheap part, immediate effect once replaced.
- Dirty condenser coil. The outdoor unit cannot reject heat. Head pressure climbs, efficiency falls, and in extreme cases the compressor trips on thermal overload.
- Failing compressor. The expensive one. Often announces itself with a breaker trip or a locked-rotor hum before it goes entirely.
- Thermostat or control fault. The system is not actually being asked to cool, or the sequence is not completing. Easy to rule out, and worth ruling out early.
How a technician diagnoses it
- Confirm the call for cooling
Check that the thermostat is closing the circuit and the system is being asked to run.
- Measure airflow
Filter condition, static pressure across the coil, blower speed. Rules the most common cause in or out quickly.
- Read the refrigerant side
Superheat and subcooling tell the technician whether charge is correct, low or overcharged, without guessing.
- Test the electrical side
Capacitor microfarads against the rating, contactor condition, motor amp draw against nameplate.
When to shut the system off
- Ice on the coil or line set. Running it longer risks liquid refrigerant reaching the compressor.
- A burning smell from the vents. Shut the breaker and call rather than waiting it out.
- The breaker trips on every cycle. Leave it off.
- Water pooling at the air handler near electrical components.
Service pages
Related guides
Safety first
Call now, or call the utility first?
Call now
- No cool air during a heat advisory
- No heat with outdoor temperatures below freezing
- Burning smell from the vents
- Water pooling at the air handler
- Ice on the line set
- Breaker tripping on every cycle
Call (855) 321-3194 and give your ZIP code.
Shut it off and call the utility or 911 first
- You smell gas: leave the building, then call the gas utility or 911
- Smoke or sparking at the equipment
- A carbon monoxide alarm is sounding: leave and call 911
Reference: CPSC recalls and US Fire Administration heating fire safety.
Questions
AC not cooling questions
Why is my AC running but not cooling?
The three usual answers are restricted airflow, low refrigerant charge from a leak, or a failed capacitor stopping the compressor from starting. A technician separates them with a static pressure reading and a superheat measurement in under an hour.
Should I turn the AC off if it is not cooling?
Yes, particularly if you see ice on the coil or line set. Running a system with a frozen coil risks liquid refrigerant reaching the compressor, which turns a modest repair into an expensive one. Let it thaw fully first.
How much does it cost to fix?
It depends which cause it turns out to be. Filter and airflow work runs a figure the provider confirms on site a capacitor a figure the provider confirms on site a coil clean a figure the provider confirms on site and refrigerant leak repair a figure the provider confirms on site. The provider confirms on site.
Can a dirty filter really stop cooling?
Yes. A loaded filter starves the evaporator coil of airflow, so the coil runs colder than designed and eventually ices over. Once iced, cooling stops entirely. It is the most common cause and the least costly to correct.
Is low refrigerant normal over time?
No. A correctly sealed system holds its charge for its service life. If charge is low there is a leak, and adding refrigerant without finding it means paying for the same gas again next season.
How long should my AC run to cool the house?
In mild weather a system typically runs in cycles of ten to twenty minutes. On the hottest days long run times are normal. A system that never cycles off at moderate outdoor temperatures is a fault.
Written and fact-checked by the HVAC Emergency USA editorial team. Last updated . Editorial policy.