Why Isn’t My Window Air Conditioner Blowing Cold Air?

When temperatures rise, there are few things as frustrating as a window or room air conditioner that suddenly stops cooling. The problem is sometimes simple enough to fix by yourself, but it may also be that your window AC has seen better days, and it’s time for you to replace it. Between these two extremes, the malfunction may be too complicated to DIY but may be fixable by a licensed HVAC technician.

If your window AC is not doing its job, you’re going to suffer when the dog days of summer are upon you. Remember that air conditioners dehumidify while they cool, so if you’re experiencing humid weather, the loss of your window AC is a double whammy. You don’t have to settle for it. Here are some troubleshooting techniques to set things right.

Identify the Problem

We’re working on the assumption your window air conditioner is getting power, and it’s either blowing warm air or no air at all. If your window AC unit won’t turn on, there’s a good chance the issue lies in the outlet or the circuit itself. Also, consider how the unit was installed, and how it sits in the window. If you can see daylight, then the hot outside air may be counteracting the cold air from the AC.

Once you determine the problem is with unit itself, there are four major areas where something can go wrong: the air flow, the temperature control, the control panel and the cooling system. Of these, the air flow and temperature control are easiest to troubleshoot.

Air Flow

Luckily, the most common source of window AC unit trouble is also the easiest to fix. Air flow is controlled by a fan and blower that draws air from your home across a filter and over cooling coils, before being pushed back out into your room.

Your first step should always be to examine the air filter and make sure it’s clean. A dirty filter can block air from the evaporator coils, never allowing cold air to circulate around the room. The filter is usually behind the front grill, and you access it from the side or top of the unit.

Also, look at the rear of the unit. Debris or even trash may collect on the casing intakes (usually on the far end of the side), causing the fan to struggle to do its job. Clear off any debris and check if something is preventing the fan blades from turning. If so, you may need to open the case to clear an obstruction. Simply cleaning an air conditioning window unit including of the filter, coils and fan will fix a surprising number of cooling issues.

If air flow seems to be okay but the unit isn’t blowing air, or it’s blowing warm air, the next item to check is the temperature control.

Why a window AC doesn’t blow cold air

“If your AC is not blowing cold air, the refrigerant levels are probably low,’ says HVAC pro Glenn Gault. “Low refrigerant levels are often indicated by ice on the condenser and in the pan.” Suspect this problem only after confirming that the filters are clean and aren’t impeding air flow. You should also check the outdoor portion of the unit, where the condenser is located. As mentioned above, leaves and other debris collecting there could well be the problem, says Gault.

Temperature Control

The temperature on a window air conditioner is controlled by the thermostat (the setting on the outside of the unit) and the thermistor. A thermistor is a thermal resistor that will turn off the system when the air near the evaporator coil reaches the temperature indicated on the thermostat. The thermistor on most window AC units is behind the air filter next to the evaporator coil.

To test the temperature control, use a multimeter to check continuity and then cool the thermistor (usually done by applying an ice cube). If the temperature control is running properly, the continuity should turn off. A damaged thermistor is relatively easy to replace; refer to your manufacturer’s documentation to find the specs for the replacement.

If the temperature control is functioning and there is no impediment to the air flow, focus on the control panel and compressor. Don’t forget to check out our list of the best window air conditioners.

Control Panel and Cooling System

The control panel is the electrical “brain” of the window air conditioner, while the compressor and refrigerant are what actually do the work of removing heat from your home. If the control panel or compressor are malfunctioning, you’re facing a significantly more complicated repair. It’s still possible to do it yourself, but it would take a more detailed walk-through and tutorial. Keep in mind that you need Environmental Protection Agency (EPA) certification to purchase or use the refrigerant itself, which is a further complication.

For these reasons, it’s best to let a pro do the work. But before you pick up the phone, says Gault, do the math. Having a HVAC technician come out to repair your window AC will likely run $150 to $250. (If you take it to a service center, the range will be closer to $75 to $200.) Depending on the size and age of the unit, it may make more sense to replace it than to sink time and money into a repair.

About the Expert

• Glenn Gault operates Gault Heating & Cooling, an Ohio-based, family-owned business established by his father in 1955.

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