One of the most common reasons for a window air conditioner to fail to deliver cold air is incorrect mode selection. However, when the unit is properly set to "Cool" but it doesn't blow cool air, possible problems include low refrigerant, dirty air filters and coil freeze-over. It's also possible the condenser motor isn't working or isn't getting power. Problems with the condenser and with refrigerant levels need to be fixed by a service pro, but homeowners can usually take care of dirty filters, freeze-over and power problems themselves.
How an Air Conditioner Produces Cold Air
When you turn on your window air conditioner and select "Cool," the condenser switches on and circulates refrigerant through a series of coils. In the first leg of its journey, the refrigerant gets compressed in the condenser coils. Midway along its path, it passes through a tiny aperture and enters the evaporative coils, where it turns into a gas. The transition from a liquid to a gas sucks heat out of the air, and as a result, the evaporative coils turn cold. A circulation fan blows this cold air out of the front of the unit.
Air Is Blowing, but It Isn't Cold
If you can feel air blowing from your air conditioner, it means the fan is working, but the condenser may not be. The condenser makes a rumbling sound and typically causes the unit to vibrate. If you don't hear the rumble or feel any vibrations, something is wrong, but the problem may be simple to fix.
Check the settings on the front of the unit. The fan selector should be set to "Auto." If it's set to "On," it will blow air even if when the condenser isn't working, but in the "Auto" position, it will only come on when the condenser is working. After correcting this setting, turn the thermostat to its maximum cold position. This should make the condenser start, and you should start to feel cold air.
If you don't feel cold air even when the condenser is working, the coils may have frozen over, the filter may be dirty or the refrigerant charge may be low. A simple procedure can help determine which of these conditions is responsible.
Getting to the Root of the Problem
Turn off the unit; open the panel on the front, and remove the air filter. It may be dirty, and if so, clean it with soap and water or buy a new one. While you're at it, go outside and clean the exhaust grid with a garden hose. Remove anything that might be interfering with air flow, such as tree branches.
Look inside the the housing with a flashlight to inspect the coils before you replace the air filter. If they are dirty, clean them with a vacuum cleaner. If they have a coating of ice, leave the air conditioner off until the ice melts. This may take a day or more. Paradoxically, a coating of ice prevents the unit from blowing cool air by forming a barrier over the evaporative coils.
Turn the air conditioner back on after cleaning the filter and deicing the coils. If it still doesn't blow cold air, the refrigerant charge may be low or the condenser may have a worn seal. At this point, you need to call a licensed service professional. It's against the law for an unlicensed person to service the refrigeration system in an air conditioner.
No Air Is Blowing at All
If no air is coming from your air conditioner, check the circuit breaker, which may have tripped. Give the plug a push to make sure it's seated in the socket. If LEDs on the front panel are illuminated, indicating the unit has power, the problem could be a stuck fan blade, or it could be a problem with the control panel. Either way, your best bet is to call a service pro to diagnose and repair the problem.