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AC not cooling: what to check before you call for service

Many no-cool calls turn out to be a thermostat setting, a tripped breaker or a clogged filter. Here's how to check the easy things safely, and when to stop.

Updated · 7 min read

  • DifficultyEasy
  • Time15–30 minutes (plus thawing time for a frozen coil)
  • If you need helpHvac technician

It's hot, the AC is running, and the house just isn't getting cooler. Before you panic about the cost of a new system, know that many "no cool" problems come from simple things: a thermostat setting, a tripped breaker, a dirty filter or a blocked outdoor unit.

The checks below take 15 to 30 minutes and are safe for any homeowner. If your system has frozen up, add a few hours for the ice to melt.

There's a clear line, though. Anything involving refrigerant, electrical parts inside the units or sealed components belongs to a licensed HVAC technician.

What you'll need

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Check the thermostat

  1. Set the mode to Cool. If it's on Heat, Off or Fan, the AC won't cool.
  2. Set the fan to Auto. If it's set to On, the fan keeps blowing even when the AC isn't cooling, which can feel like warm air.
  3. Lower the setpoint. Set it a few degrees below the room temperature.
  4. Check the screen. A blank display may mean dead batteries. See our guide to a thermostat that's not working.

Give the system a few minutes to start. Many systems wait briefly before the outdoor unit turns on.

Check the breaker and outdoor disconnect

A central AC system usually has two parts: an indoor unit (a furnace or air handler) and an outdoor unit. Each typically has its own breaker. If the indoor fan runs but the outdoor unit is silent, the outdoor unit may have lost power.

  1. Check your panel. Look for breakers labeled "AC," "condenser" or "furnace." Reset any that are tripped. See how to reset a tripped breaker.
  2. Check the outdoor disconnect. Most outdoor units have a small box mounted on the wall nearby. Inside is a switch or pull-out block. Make sure it's in the on position and fully seated. Don't open or touch any wiring inside.
  3. Watch for repeat trips. If a breaker trips again, leave it off and call a technician.

Check the air filter

A clogged filter is one of the most common causes of poor cooling. It chokes off the air, and it can even make the indoor coil freeze. The Department of Energy recommends cleaning or replacing the filter every month or two during the cooling season, and more often if you have pets or dust.

Pull the filter out and hold it up to a light. If you can't see light through it, replace it. Our guide on how to change your HVAC filter walks through it.

While you're at it, walk through the house and make sure supply vents and large return grilles aren't blocked by furniture or rugs.

Clear the outdoor unit

The outdoor unit pushes heat out of your home. It needs open space to breathe. The Department of Energy suggests trimming plants back at least 2 feet around it.

  1. Turn off power. Use the thermostat and the outdoor disconnect or breaker.
  2. Remove debris. Pull leaves, grass clippings, cottonwood fluff and trash off the sides and top by hand while wearing gloves.
  3. Trim plants back. Keep about 2 feet clear all around.
  4. Look at the fins. If they're caked with dirt, a technician can clean the coil during a service visit.
  5. Restore power and listen. The fan on top should spin and you should feel warm air blowing up.

If you see ice, turn the system off

Ice on the refrigerant lines near the outdoor unit or on the indoor coil means the system has frozen. Carrier says to turn it off right away so the ice can thaw, since running it frozen can cause more damage.

  1. Turn cooling off at the thermostat.
  2. Let it thaw on its own. Carrier says this may take a few hours or even overnight. Don't chip the ice or use a hair dryer or heater on it.
  3. Watch for melt water. Put towels around the indoor unit in case the drain pan overflows.
  4. Replace the filter. Airflow problems are a common cause of freezing.
  5. Restart and watch it. If it freezes again, call a technician. Carrier notes that low refrigerant from a leak can also cause freezing.

Check the condensate drain

As your AC cools, it pulls water out of the air. That water drains away through a small pipe near the indoor unit. If the line clogs, Trane explains that a safety switch on many systems shuts the AC down to prevent an overflow.

  1. Look at the indoor unit. Check for water in the drain pan, a wet floor or a drip line that's stopped dripping outside.
  2. Clear the end of the line. Find where the pipe ends outdoors or at a floor drain. With the system off, attach a wet/dry vacuum to the end and run it for a minute or two to pull out the clog.
  3. Dry the pan and restart. If the switch reset, the system should start again.

The Department of Energy also notes that clogged drains can leave water stains on walls or carpet, so don't ignore drips.

When to call a pro

Call a licensed HVAC technician if:

  • The coil freezes again after you've thawed it and replaced the filter.
  • You see oily residue on refrigerant lines or hear hissing. Those can be signs of a leak.
  • The outdoor fan doesn't spin, or the unit hums, clicks or buzzes but won't start.
  • A breaker trips again after reset.
  • Everything checks out and the house still won't cool.
  • You rent. Report it to your landlord or property manager.

Refrigerant work is not a DIY job. Under EPA rules, refrigerant for home AC systems is sold to technicians who hold Section 608 certification. A technician also needs to find and fix any leak, not just add more. Read how to choose an HVAC technician, or take our fix it or call a pro quiz.

Keep it from happening again

  • Check the filter monthly during summer and replace it when it looks dirty.
  • Keep plants, fences and clutter at least 2 feet from the outdoor unit.
  • Have the system serviced before the cooling season starts. Our spring home maintenance checklist can remind you.
  • Keep vents and return grilles open and unblocked.
  • Ask your technician to check the condensate line and drain pan during service.

Common questions

Why is my AC running but blowing warm air?

Common causes are a thermostat set to fan or heat, a tripped breaker for the outdoor unit, a dirty filter, a blocked outdoor unit or a frozen indoor coil. If those all check out, low refrigerant or a failed part may be the cause, and that needs a technician.

What should I do if my AC coil is frozen?

Turn the cooling off right away and let the ice melt on its own. Carrier says thawing may take a few hours or even overnight. Don't chip at the ice or use a heater on it. Replace a dirty filter, and call a technician if it freezes again.

Can I add refrigerant to my AC myself?

No. Under EPA rules, refrigerant for home air conditioners is sold to technicians with Section 608 certification. Low refrigerant also means there's a leak, which a technician needs to find and fix.

How often should I change my AC filter?

The Department of Energy suggests cleaning or replacing it every month or two during the cooling season, and more often if you have pets or a lot of dust. See our guide to changing your HVAC filter.

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