Quick answer
- Start with the simplest fix: a clogged filter restricts airflow
- Ice on the lines points to airflow or low refrigerant
- Warm air outside the unit but warm air inside = refrigerant or compressor
- Persistent issues need a professional diagnosis to avoid compressor damage
When this applies
Try this first
Replace a dirty filter and make sure vents are open and the outdoor unit is clear of debris. If that doesn't restore cooling within a few hours, it's time for a diagnostic.
How it works
How cooling actually works
Your AC moves heat out of your home using refrigerant. Anything that blocks airflow (filter, coil) or reduces refrigerant lowers cooling capacity and stresses the compressor.
What to watch out for
Frozen evaporator coil
Restricted airflow or low refrigerant makes the coil freeze. Running the system while frozen can damage the compressor — shut it off and call for service.
Low refrigerant
Refrigerant doesn't get 'used up' — if it's low, there's a leak. Topping it off without fixing the leak is a temporary, costly band-aid.
Schedule service
Frequently asked questions
Why is my AC blowing warm air?
Common causes are low refrigerant, a tripped breaker on the outdoor unit, a frozen coil, or a failing compressor. A technician can pinpoint it quickly and safely.
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Reviewed by John Stevenson Plumbing, Heating & Air. Based on the most common no-cool service calls our NATE-certified technicians diagnose in the field. Last updated 2026-06-18.