When temperatures rise, there are few things as frustrating as a window or room air conditioner that suddenly stops cooling. If your window AC is not doing its job, 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, accessed 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.

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, further troubleshooting will 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. Environmental Protection Agency (EPA) certification is required to purchase or use the refrigerant itself.

For these reasons, it’s best to let a pro do the work. But before you pick up the phone, do the arithmetic. 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, that 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 sink time and money into a repair.