Window Air Conditioner Keeps Running When Turned Off?

Off means off, right? Sometimes your window AC doesn’t agree. It keeps on running. Why?

Sometimes window air conditioners have electrical or mechanical faults that cause the unit to run indefinitely. That issue raises energy costs and lowers temperatures to uncomfortable degrees, literally.

So let’s solve this.

Why Does my AC Keep Running When I Turn if Off?

Here are a handful of common malfunctions that might cause a window AC to stay running. Some faults are fixable, but some require a professional to handle the repair. Here’s a list in order from the easiest to most difficult fixes:

  • Dirty Coils
  • Too Hot Outside
  • Electrical Control Board (PCB) is Bad
  • Temperature Control or Thermistor = Temperature Control System is Bad

Summary of Common Reasons an AC Keeps Running When Turned Off

Let’s jump right into the causes. A problem involving the board is bad – fixable but probably at a cost near the price of a new window air conditioner. So, we’ll explore other options too. Try them first, and see what happens before you trash the AC and head to Amazon or Home Depot for a new window air conditioner.

The other issues on the list can be solved. The thermistor and dirty coils can typically be fixed with the help of a YouTube video. If heat is the issue, the problem will resolve itself when cooler temperatures roll through.

The Air Filter and/or Coils are Dirty

This might not cause the AC to run if you turn it off, but it will result in the air conditioner running constantly. Even if you have a mechanical problem, the “clean your AC” and “start it early” tips in the first two will make you happier with the performance of your window AC.

This is a fixable hitch. Your air conditioner’s air filter might be clogged. This can lead to the coils becoming dirty – or they might just get dirty all by themselves. Debris from outside can cling to the “fins” on the back of the AC. Inside, pet and human hair and other gunk can dirty them up.

When the filter and/or coils are dirty, your AC unit won’t cool as effectively. As a result, it might just keep running and running and…it needs to be cleaned! If it doesn’t get cleaned, you probably won’t get cool. And the overtime work could cause the internal parts, especially the compressor, to overheat and be damaged. That’s the end of a window air conditioner’s life right there.

For starters, open the AC face and wipe off any excess dust grime. Use a shop vacuum to clean anything in site including the fins – the coil on the back/outside of the unit. And rather than bore you with additional steps, check out an entertaining and very useful video like this one.

Maybe it is Just Too Hot Outside (and Inside)

One of the keys to effective cooling with a window air conditioner is to get ahead of the heat. If it’s going to be sunny and 90 out, and your window AC is in a west-facing or south window, you should turn it on well before the heat of the day. If you wait, the room might simply be too hot for effective cooling.

And since part of the AC is outside the window, it can get very hot inside – even hotter than the air temperature. So, indoors, you might be chilly. But because the window air conditioner thermostat or thermistor is inside the unit, it might still think the room is hot and keep running even when your room is as cool as you want it to be.

The Electrical Control Board is Bad

Now we get into potential mechanical issues. They can be fixed, but you’ll have to decide if the fix is better than replacing the AC. We’ll help you decide. 

Electrical Control Board

Also called the PCB or printed circuit board, this piece is the control center of the AC. It’s replaceable, but probably at a cost of about half what a new window air conditioner would cost. The part is cheap; it’s the hour or more of labor that is costly. If you’re handy, look for a DIY fix online plus the part, and give it a shot.

Here’s what happens. Your AC has a thermostat or thermistor that monitors temperature. When your AC temperature set point is reached, the thermostat or thermistor signals the circuit board. Output relays on the PCB then function to shut off the compressor and put the fan into a short cycle to expel the rest of the cool air and then shut off.

The snag comes when the relays don’t work as they should. The AC just keeps on running even when it should be shutting down. Relays not replaceable. You’d have to replace the entire board at a parts cost of $35 to $70 depending on the model.

The Temperature Control System is Malfunctioning

The next two items address the window air conditioner’s ability to properly sense the temperature in the room. If it can’t do that, the system won’t shut down once the desired temperature – the temperature you set on the AC – is reached. These aren’t easy to diagnose. If the “easier” fixes above don’t work, you might not even want to get into this subject. Instead, you might prefer to take your AC to a repair shop, have a repairman to your home if you can find one that will come – or just get a new window air conditioner.

If you’re in the market for a new window air conditioner, see our Window AC Unit Reviews and Buying Guide first. Or choose the Window AC tab at the top to begin researching the many, comprehensive posts we’ve provided on the type and size window air conditioner you want.

OK, first we’ll discuss the temperature control system as a whole and then turn to the thermistor/thermostat as an isolated part.

The Complete Temperature Control System

The group of parts in an AC unit that reads the temperature in the room and tells the compressor when to turn on and off is called the temperature control system. A sensing bulb and capillary tube monitor the temperature. When the room is warm, they  tell the compressor when to kick on. When cool enough, they signal the AC to turn off.

Take a look: If the temperature control is causing your window AC unit to run constantly, you will be able figure it out by checking the sensing bulb. Be sure the sensing bulb isn’t broken or disconnected. If you can’t determine that the sensing bulb is the issue, a professional will need to take a deeper look.

The Thermistor or Thermostat

The thermistor looks like this.

window air conditioner Thermistor

Here’s a window air conditioner thermostat.

window air conditioner thermostat

Your unit has one or the other, but not both.

These are housed at the inward-facing side of the AC unit. You’ll have to remove the grill and cover of the window air conditioner to find it. Sometimes there is simply a problem with the connection between the thermistor or thermostat and the control panel or electrical control board. You can deduce this problem with a multimeter by checking the ohms of the thermistor. If no power is going through it, the part should be replaced. It’s a fairly easy DIY job. Just make sure the new part you buy is suitable for the AC model you have.

DIY or Pro?

If you have time to tinker, tools and a multimeter, you can snoop around to see if the temperature sensing parts are bad. Does the PCB look burned? There’s a clue. How about dirt? That can be dealt with.

You might be able to diagnose the issue without paying a service repair tech. If not, expect to pay between $40 and $75 just for the diagnostic fee which should cover basic cleaning and about 1 hour of labor if repairs are needed. The upside is that performing maintenance can prevent most of these faults from taking place.

The bottom line is that if your window AC is already 8-10 years old or older, our pro recommendation is to put your money into a new unit. Today’s window air conditioners are quite efficient, especially those that are Energy Star rated, and have lots of features you’ll enjoy like remotes, remotes with temperature sensors in them, timers, dehumidifier mode and others that weren’t available just a decade or so ago.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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