AC running but blowing warm air

When the AC runs but the vents blow warm air, the cause is usually a setting, a clogged filter, or a problem at the outdoor unit. Five quick checks at the thermostat, the filter and the outdoor unit sort out what you can fix and what needs a technician.

Also searched as: ac blowing warm air, ac blowing hot air, ac running but not cold, ac blowing room temperature air.

Before you start. Carrier advises turning the AC off if it keeps blowing warm air, because running a malfunctioning system can overheat the compressor.

A family relaxing on a sofa in a bright, cool living room

Every question and answer on this page, in order

Question 1: What is the thermostat set to?

Look at the mode, the fan setting and the set temperature, not just the number on the screen.

Cool, fan on Auto, a few degrees below room temperature: go to question 2
Heat, Fan, or the fan set to On: Likely cause: a thermostat setting

Question 2: Is the outdoor unit running?

Go outside while the thermostat is calling for cooling. Listen for a steady hum and look down through the top grille at the fan.

Yes, it hums and the top fan is spinning: go to question 3
No, it's silent or the fan isn't spinning: Likely cause: the outdoor unit isn't running

Question 3: Do you see ice or frost on the copper line or the indoor unit?

Check the larger copper line where it leaves the outdoor unit and where it enters the indoor unit. Look only; don't open any panels.

Yes, there's ice or frost: Likely cause: a frozen coil
No ice anywhere: go to question 4

Question 4: How does the air filter look?

Slide the filter out of the furnace, air handler or return grille and hold it up to a light.

Gray or dusty, and little light comes through: Likely cause: a clogged air filter
Clean, or changed in the last month: go to question 5

Question 5: Which of these do you notice?

Leaves, grass or shrubs crowding the outdoor unit: Likely cause: a blocked outdoor unit
Water in or around the indoor unit or its drain pan: Likely cause: a clogged condensate drain
A hissing sound, or oily residue near the unit or lines: Likely cause: low refrigerant or a failed part
None of these: Likely cause: low refrigerant or a failed part

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.