Why Is Air Conditioner Not Cooling? (9 Causes & Fixes)

Your AC is running but the room isn't getting cool. Before calling a technician, work through these common causes — most can be fixed yourself in minutes.

1. Dirty Air Filter

Most common cause. A clogged filter restricts airflow over the evaporator coil, dramatically reducing cooling capacity. The coil can't absorb heat from the air if air can't reach it.

Fix: Check the filter. If it's grey and clogged with dust, replace or clean it immediately. Replace every 60–90 days during heavy use. The Amazon Basics MERV 8 Air Filters (6-Pack) are a reliable, affordable option for standard units.

2. Thermostat Set Too High

Check that the thermostat or remote is set to Cool mode and the target temperature is lower than the current room temperature. If the set temperature equals or exceeds the room temperature, the compressor won't run.

Fix: Set the temperature at least 5°F below the current room temperature and confirm the mode is Cool, not Fan or Auto.

3. Dirty Evaporator or Condenser Coils

Coils coated in dust and grime can't transfer heat efficiently. The AC runs but barely cools.

Fix: Clean the coils with AC coil cleaner and straighten any bent fins with a fin comb. See our full guide on how to clean AC coils.

4. Low Refrigerant (Freon Leak)

Refrigerant is the substance that actually transfers heat. If the system has a leak, refrigerant levels drop and cooling capacity falls significantly. Signs include ice forming on the indoor unit, hissing sounds, and warm air from vents despite the compressor running.

Fix: This requires a licensed HVAC technician. You can use a refrigerant leak detector to confirm a leak before calling, but recharging refrigerant must be done professionally.

5. Blocked Outdoor Unit

The outdoor condenser unit needs clear airflow to release heat. If it's surrounded by vegetation, debris, or has a cover still on from winter, it can't dissipate heat and cooling performance drops.

Fix: Clear at least 2 feet of space around the outdoor unit. Remove any covers. Rinse the condenser coils with a garden hose if they're dirty.

6. AC Unit Too Small for the Room

An undersized AC will run continuously without ever reaching the set temperature in a large or poorly insulated room.

Fix: Check the BTU rating against your room size. As a guide: 5,000 BTU for 150 sq ft, 8,000 BTU for 350 sq ft, 12,000 BTU for 550 sq ft. If the unit is undersized, consider upgrading or adding a supplemental unit.

7. Doors and Windows Open

Cool air escaping and warm air entering will prevent the room from reaching the set temperature no matter how hard the AC works.

Fix: Close all doors and windows while the AC is running. Seal gaps with door draft stoppers and window sealing tape.

8. Compressor Not Running

If the fan runs but you don't hear the compressor (a deeper humming sound from the outdoor unit), the compressor may have failed or tripped a safety switch.

Fix: Check the circuit breaker for the outdoor unit. Reset if tripped. If the compressor still doesn't start, call a technician — compressor issues require professional diagnosis.

9. Clogged Drain Line

A blocked condensate drain can cause the AC to shut down via a float switch, or reduce efficiency by causing the drain pan to overflow onto the evaporator coil.

Fix: Clear the drain line with AC drain line cleaner. See our guide on how to clean the AC drain line.

Quick Diagnostic Checklist

• ☐ Filter clean?
• ☐ Mode set to Cool, temperature below room temp?
• ☐ Coils clean?
• ☐ Outdoor unit clear of obstructions?
• ☐ All doors and windows closed?
• ☐ Circuit breaker not tripped?
• ☐ Drain line clear?

Work through this list before calling a technician. In most cases, the fix is a dirty filter or blocked outdoor unit — both solvable in under 30 minutes.