When your AC stops cooling, the fix is sometimes a 30-second thermostat check and sometimes a failed compressor. This guide walks you through the most common reasons an air conditioner stops cooling — the thermostat, the air filter, the capacitor, the refrigerant level, and the compressor — and tells you exactly when to stop troubleshooting and call a licensed HVAC pro through AutoReach.
These eight causes account for the overwhelming majority of "AC not cooling" service calls. Click any card to jump to the matching DIY check or the warning signs that mean it's time to call a pro.
The system is set to HEAT instead of COOL, the fan is locked to ON, or the batteries are dead and the thermostat is reading a phantom temperature. Check the thermostat →
A neglected filter starves the indoor coil of airflow, the coil freezes, and the system blows warm air even though the outdoor unit is running. Swap the filter →
Leaves, dirt, a fallen branch, or a neighbor's fence leaning into the unit block the heat the refrigerant needs to dump. The compressor overheats and protects itself by stopping. Clear the condenser →
Ice on the indoor coil (or copper lines at the air handler) means airflow can't keep up — usually from a dirty filter, weak blower, or low refrigerant. Don't keep running it →
Refrigerant isn't "used up" — if it's low, there's a leak in the lines, coil, or fittings. A pro has to find and repair the leak before recharging. Find a pro →
The capacitor gives the compressor and outdoor fan motor their starting jolt. A failed one often produces a humming outdoor unit and a silent indoor blower. Pro replacement →
The contactor is the heavy-duty relay that sends power to the compressor and outdoor fan. Pitted or stuck contacts prevent the outdoor unit from starting even when the thermostat calls for cool. Pro replacement →
The compressor is the heart of the system. On units 12+ years old, a failed compressor often means it's time to weigh repair against replacement. Pro replacement →
Before you call for AC repair, run through this list. About a third of "AC not cooling" tickets turn out to be one of these — and your fix may only cost a filter or a tripped breaker.
Some warning signs mean the AC has crossed from a minor homeowner fix into a refrigerant, electrical, or sealed-system problem. Continuing to run the system in these cases can turn a $300 capacitor swap into a $4,500 compressor replacement.
If a brand-new filter didn't restore cooling and the system is still blowing room-temperature air, the problem is downstream of airflow — usually low refrigerant or a failing compressor.
A hiss or bubble near the indoor unit or outdoor condenser means refrigerant is escaping the sealed system. The system has to be leak-tested, repaired, and recharged by an EPA-certified tech.
If the indoor coil won't thaw with the fan running, the underlying cause isn't airflow — it's low refrigerant from a leak, and the compressor is at risk of hydraulic damage every minute it tries to start.
One tripped breaker is normal; two in the same day is a short, ground fault, or locked compressor. Resetting a tripping breaker is a fire risk — leave it OFF and call for AC repair.
A sweet, ether-like smell is refrigerant leaking into the airstream. Shut the system down, ventilate the home, and call a pro — refrigerant exposure is a health hazard and the leak needs to be sealed.
On a unit past 15 years, a failed compressor or coil is often a sign the system is at end of life. A pro can quote both the repair and a like-for-like replacement so you can make the call with numbers, not guesses.
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