Solved! What to Do If Your AC Stops Cooling

Don’t sweat it if your central air conditioning stops cooperating. Troubleshoot the most likely problems with these wise moves.

Troubleshooting AC Unit


Q: I’ve just turned on the air conditioner for the summer, but have yet to get comfortable. The appliance circulates air, but nothing refreshingly chill. Why is my air conditioner not cooling? More importantly, how do I fix it?

A: It’s the last thing you want on a sweltering summer’s day—a central air conditioning system that’s not blowing cold air. While your first reaction may be to contact an HVAC pro, with a little of your own troubleshooting, you might remedy the problem and save on a costly house call.

Air conditioning systems operate on a basic scientific process called “phase conversion.” Refrigerant, the liquid used in an AC system, undergoes a continuous cycle of evaporation and condensation within the unit’s sealed coil system. The unit’s evaporative coils (usually located inside your home near a blower unit) become icy cold as the refrigerant within turns from a liquid to a gas. The unit’s fan blows air over those icy coils, which forces cooled air through your home’s ducting. The gas then cycles back to a condenser coil unit (located outside) where it cools back down to a liquid and the cycle repeats itself over and over.

If your AC system is blowing warm air, several culprits may be to blame.

It might be time to call a pro
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Check the thermostat.

It may seem simplistic, but sometimes the cause of an AC system running but not cooling is simply the result of someone switching the thermostat from “Automatic” to “Fan.” When the switch is set to “Automatic,” the thermostat, which is an integral part of the system, switches on the air conditioning when the indoor temperature rises above the desired temperature that you’ve pre-set. If the switch was accidentally set to “Fan,” however, the unit will blow air through the duct system, but no cooling will take place. Check and reset the switch to “Automatic,” if necessary.

Replace a dirty filter.

If it’s been more than a couple of months since you’ve replaced the return-air filters in your AC system, they may be clogged and dirty, which can affect air flow. When filters get clogged with animal fur and dust, the AC system can’t draw in sufficient air, and as a result, only a wimpy flow of air comes out. So remove a return-air filter and, if you can’t see what’s on the other side, replace it. if you can see through the filter, the problem lies elsewhere.

Clear a clogged condensation drain.

Air conditioners work in part by removing humidity from the air (through condensation), and that moisture must go somewhere. That’s the job of a condensation drain hose—it directs water to a floor drain or to the outside of your home, depending on your system. Condensation drains are subject to blockage by mold and algae growth. When this happens, some AC units will stop cooling while others will shut down completely.

Locate the end of the condensation drain line (often in a utility room) and visually check it for clogs. If you see a clog, carefully clear it out with the end of a small screwdriver or similar narrow item. If a clog forms higher in the line where you can’t physically reach it, suction on the end of the line will usually remove it. Use the hose on a wet/dry shop-type vacuum—and hold your hands around the opening—to create sufficient suction between the two hoses. After removing a mold or algae clog, pour a couple of cups of white vinegar into the condensation pan that lies beneath the evaporator coils in the inside blower unit (learn how to access and identify the coils and the condensation pan below). The vinegar will kill residual mold buildup and reduce the risk of future clogs.

Troubleshooting AC Unit


Discern a duct malfunction.

In a central AC system, the main blower forces cold air through the ducting and from there into individual rooms. If a duct somewhere between the blower and a room register (the metal grill that covers the opening of an HVAC duct) has broken, the cold air could be blowing out before it reaches the room’s register. If cool air is blowing from some registers but not from others, there’s a good chance the ducting that feeds the registers is at fault. If you have a basement, you can examine the ductwork to see if a joint has come loose. If so, refit the ends of the joint and tape the new joint securely with duct tape. If a ducting joint has come loose within a wall, however, you won’t be able to locate it and will need to call an HVAC professional.

Clear the compressor area.

If dry leaves and debris have piled up next to the compressor unit, it may not be able to draw in sufficient air. To find out, locate the compressor unit, which will typically be tucked away on the back or the side of the house where it won’t draw attention. Sometimes, a small fence may have been installed around it to keep it from detracting from the rest of the landscape. Clean away all debris or anything else that might be crowding the unit, such as overgrown vines, and don’t place anything on top of it for peak functioning.

Get serious with dirty coils.

The typical AC system has two sets of coils—condenser coils, which are located in the outside compressor unit and evaporator coils, which are encased near the indoor blower unit. When either set of coils becomes dirty or covered with mold and debris, cold air output can suffer. Cleaning the coils involves removing the metal enclosures that protect them.

If you don’t feel comfortable working inside the units, it’s time to call a pro, but if you’d like to try cleaning the coils on your own:

  • Shut off the power to both the exterior and interior units at the breaker panel. Each one will be on a separate breaker.
  • Follow the AC manufacturer’s directions for removing the exterior compressor cage or the metal panels that house the evaporator coils.
  • To clean interior (evaporator) coils, spray a non-rinse evaporator coil cleaner (view on Amazon) directly on the coils, which resemble U-shaped copper or steel tubes. The non-rinse cleaner foams up on the coils and dissolves dirt and grime before liquefying and running into a condensation pan that empties into the condensation drain hose.
  • To clean exterior (condenser) coils, spray the coils, and the thin metal fins that surround them, with a condenser coil cleaner (view on Amazon). This cleaner is different from evaporator coil cleaner and it will require rinsing with the hose. Follow the product directions carefully.

Know when to call the HVAC pro.

If you’ve gone through the above DIY steps and your AC system is still not cooling, the problem could be leaking refrigerant (Freon) or a failed compressor unit. Freon is federally regulated and may only be handled by a licensed HVAC professional. A failed compressor, especially if your AC system is more than 10 years old, may signify the need to purchase a new system. These issues must be addressed by the pros, so make the call!

It might be time to call a pro
Get free, no-commitment repair estimates from licensed HVAC technicians near you.