An AC that runs but doesn't cool is one of the most frustrating home problems in summer. The most common causes are all DIY-checkable in 15 minutes: dirty filter, frozen evaporator coil, blocked condenser, or incorrect thermostat setting.
Likelihood: Most common cause
A severely clogged filter restricts airflow so much that the system can't move enough air to cool effectively — and may freeze the evaporator coil.
Fix: Replace the filter ($5–25 for a quality filter). Do this first.
Likelihood: Very common
Ice forming on the indoor coil completely blocks airflow. The AC runs but no cool air comes out. Often caused by a dirty filter or low refrigerant.
Fix: Turn AC to fan-only for 2–3 hours to defrost. Fix the root cause (filter or refrigerant) before cooling again.
Likelihood: Common
The outdoor unit needs clear airflow. Debris, grass clippings, or a dirty coil prevent heat rejection, reducing cooling capacity.
Fix: Clear 2 feet of clearance around the condenser. Rinse fins gently with a garden hose.
Likelihood: Requires professional
Refrigerant doesn't deplete normally — a low charge means there's a leak. Symptoms: ice on coil, hissing sound, warm air from vents.
Fix: Must be diagnosed and recharged by a licensed HVAC technician.
Likelihood: Surprisingly common
Fan mode circulates room-temperature air. Verify the mode is set to COOL and the set temperature is below the current room temperature.
Fix: Set thermostat to COOL and set temperature 3–5 degrees below current room temp.
Confirm mode is set to COOL (not FAN or HEAT). Set temperature 3–5 degrees below the current room temperature. Replace the thermostat batteries if it's battery-powered.
Find your air handler (typically in a closet, attic, or basement). Slide out the old filter and check the size printed on the frame. Install a new filter with the arrow pointing toward the air handler.
Tip: Use a MERV 8–11 filter for good filtration without over-restricting airflow. Higher MERV ratings can choke smaller systems.
If the filter was severely clogged, the evaporator coil may be iced over. Look at the refrigerant lines going into the air handler — are they coated in ice? If yes, turn the system to FAN ONLY for 2–3 hours to defrost before running cooling again.
Turn off power at the disconnect box next to the outdoor unit. Clear any debris from the top and sides. Gently rinse the fins with a garden hose from inside out (don't bend fins). Restore power and test.
Running without cooling to setpoint usually means refrigerant is low (leak), the coil is frozen, or the unit is undersized for the heat load. Check the filter first. If that's clean and the coil isn't frozen, call an HVAC tech to check refrigerant charge.
Legally, no — in the US, purchasing and handling refrigerant (R-410A, R-22) requires EPA Section 608 certification. Technicians must also find and repair the leak before recharging — adding refrigerant to a leaking system is just money lost.
Every 1–3 months for standard 1-inch filters. Every 6–12 months for 4–5 inch media filters. More often if you have pets or allergy sufferers in the home.
Call an HVAC technician if: you suspect low refrigerant (system must be diagnosed and charged by a certified tech), if the compressor won't run (outdoor unit is silent), or if the evaporator coil keeps freezing despite a clean filter.
ChatDIY can adapt this diagnosis to your exact fixture, tools, photos, and skill level.