How to fix a Window Air Conditioner not Cooling

Your window air conditioner is not cooling? In this article, I am listing all of the possible reasons!

I’ll tell you exactly how to test each cause and how to fix it.

The list is sorted by issue frequency. So, the most common reasons are listed first.

Quick answer: To fix a window air conditioner not cooling, ensure that all airways are free. Clean the evaporator coil. Ensure that the power supply is OK and that there is no issue with the power cord. Also, properly insulate the window setup of your window AC. If nothing helps, get a new window AC.

Reasons and fixes for a window air conditioner not cooling

Let’s have a look at the complete list of causes and fixes!

Clean clogged air filters

The most common reason for a window AC not cooling is that it can’t blow any air! And air movement is very important in an air conditioner.

The air is responsible for transporting the cold into your room.

So, if a window AC is clogged internally, it can’t blow out cold air at all. Or if it’s only slightly clogged, the window AC might look and sound like it’s working, but it won’t put out enough cold air to really make a difference.

How to check: Checking whether your air filters are clogged is very easy. Most window ACs open right on the front panel.

There, you can take out the air filter (no tools required) and see whether they are dusty. 

If your air filters are not dusty, you can further open the window AC (tools required) and see whether the internals are filled with dust.

How to fix: Remove all the dust you find in your window AC using a dry brush. Never wet air filters!

I see so many people do this (in YouTube videos)… They wet-clean their filters and then slide them right back into the AC.

Never do this.

Instead, dry brush the air filters using this air conditioner brush set (click here to view it on amazon).

( Or just use any regular brush you have at home already – old toothbrushes work perfectly well )

The reason you want to dry-brush your window air conditioner is that this takes off most of the dust, mites and bacteria without enhancing their growth.

Water and dust combined are a perfect living environment for bacteria and mold.

So, if you don’t want your air conditioner to stink after cleaning it, don’t use water to clean the air filters!

Ensure the wall outlet is working

If your window air conditioner is not only not cooling, but also not even turning on, it could be that an element of the wall outlet that your window AC is connected to is broken.

Luckily, this is easy to check.

How to test: Grab another device and plug it into the wall outlet. Does it turn on?

If it turns on, then your wall outlet is fine.

If it doesn’t turn on, then test whether your window AC turns on when connected to another wall outlet (which you know is working). In case your window AC turns on, then you found the reason.

Else, the fault is likely in your window AC’s electronic power element.

How to fix: If you found your wall outlet to not supply any power, check the fuses or circuit breakers in your electric service box.

If you find a fuse blown, exchange the fuse with a new one. In case you find a circuit breaker tripped, just switch it back to the original position.

However, if you found your window air conditioner still not working, even after connecting it to a working wall outlet, the fault is likely in the window AC’s power module.

Repairing the power module requires good electronics knowledge. You will have to find the broken part (a blown capacitor or burned resistor) and replace it with a matching part.

If you know how to do it, go ahead.

Else, better let a professional fix it. Or, actually, it could be even cheaper to just get a new window AC.

Ensure the extension cord is OK

When using a window air conditioner with an extension cord, you should always make sure that your extension cord has the proper gauge to be able to handle the window AC’s current requirement.

If you have a cheap extension cord and you don’t know for sure whether it is suitable for your window AC, you can check out my guide on extension cords for ACs.

How to test: The best way to check an extension cord is using an electric measurement device such as a multimeter. With a power multimeter, you can check whether the input power reaches the other end of the extension cord.

Also, you can check whether the cord is broken and if there’s a connection from one end to the other.

How to fix: If you detect any issue with your extension cord. Or even if you are not sure about the extension cord you have, you should get a heavy-duty extension cord.

Window ACs draw a lot of current. And most regular extension cords are not able to handle it.

Either they overheat or they increase your window AC’s electricity consumption or both.

Clean the evaporator coil

A window ACs evaporator coil should never be clogged or dirty! The evaporator coil carries heat from your room’s air to the refrigerant (which in turn evaporates and absorbs the heat – hence the name).

If your evaporator coil is dirty, then your window AC won’t cool your room, even if it’s running.

How to test: Open your window AC’s side (or front, or wherever the coil is located – cross-check with your manual).

After you locate your evaporator coil, you should be able to see whether it’s dirty.

How to fix: Use an air conditioner coil cleaning foam (click here to view it on amazon) to dissolve and brush off all of the dirt.

You can use the special brushes included with an AC brush kit. But you can just as well use any other old brush you have lying around.

Insulate the window setup properly

A window air conditioner always needs to be well-insulated in order to function properly. There should be no gaps around it!

If there are any air gaps or improperly insulated spots around your window AC, heat from outdoors will enter your room. And this will annihilate your window ACs cooling (at least partly).

So, a proper window setup always goes along with proper insulation.

How to check: The easiest way to inspect your window setup’s insulation is visually. Just inspect the area around the window AC.

Can air leak into your room anywhere?

How to fix: If you find poorly insulated spots, then you have two options. 

Most window ACs come with an insulation kit. Usually, this kit is good enough. Ask yourself whether the gaps in your setup are the kit’s fault, or whether they are caused by careless installation.

  • If you conclude that the gaps are your fault, reinstall the window AC properly and carefully. Work precisely and close any gaps.
  • If the leaks are due to the kit not properly fitting your window, then you should insulate the gaps around your window AC using an insulation foam.
    Usually, when you get a window or portable AC, you also get a small roll of insulation foam.
    Get that and install it. If you don’t have it, just buy and install your own insulation material.

This is the insulation foam roll I got with my (portable) AC:

Drain water

Window ACs absorb water from your room’s air. The water, in turn, has to go somewhere.

Now, this differs from model to model, but usually, window ACs accumulate water in the bottom part of the outside component.

Your AC might have a sensor detecting an overflow which causes a reduction in cooling. Or the overflowing water pan might inhibit the cooling in some other way.

Normally, all of the accumulated water should evaporate (through the condenser’s heat). But on very humid days, new water accumulates faster than it can evaporate.

How to test: Go around the house to the outside part of your AC and check where the water accumulates.

How to fix: Attach a drain hose to drain the water. Alternatively, you can place some gauze (acting as a wick) in the back of the AC.

Set the AC to cooling mode

Some window ACs can switch between a heating and cooling mode. In case your window unit is set to heating mode, it won’t cool your room.

How to test: Check the front panel display whether there is a heating or cooling setting available. Also, check your remote for an option. If you don’t find anything, look it up in the manual or online for your specific window AC unit.

How to fix: If your window AC is set to heating mode, simply switch it back to cooling. It should start blowing cold air immediately.

Fix the broken thermostat

Window ACs have a built-in thermostat that measures the current room temperature. Based on the room temperature, the thermostat calculates the cooling power necessary to get your room to your desired temperature.

Then, it commands the compressor.

If your window AC’s built-in thermostat is broken, either mechanically through impact (while cleaning, or the unit falling down), or electrically (through a blown capacitor, or a broken sensor), your window AC won’t cool.

How to test: Check the temperature display on your window AC. Which current temperature does it display?

If the displayed room temperature is off, the thermostat’s thermometer is broken.

Else, open your window AC, locate the electronic circuitry and look for broken parts.

It could be that the thermostat is not a separate component but integrated with other electronic components on one circuit board.

In this case, you will have to check the entire electronics and look for faulty parts.

How to fix: Replace damaged parts with matching replacement parts. You can find replacements online.

To do this, you should be familiar with electronics and ideally know how to replace soldered parts.

Window AC still not cooling?

If you still can’t figure out why your window AC is not cooling, or your electronic skills do not suffice to fix it, then I’d say just order a new unit online.

Window ACs are the most cost-effective and affordable kind of air conditioning system. And you can find good ones for not too much money.

I recommend this 12,000 BTU Midea AC (click here to view it on amazon). This model is strong enough to cool rooms up to 300 square feet.

If that’s too much for you, just choose a smaller BTU option of the same model.

But honestly, any window AC will do the job!

So, just pick one you like.

Attribution: title image by jasoneppink