When you see the “Cool on” light on your Honeywell thermostat blinks you perhaps don’t know whether it is “normal” or not.  Probably you are used to seeing it blinking from time to time but now it is flashing continuously. As this flashing is now going on for a few hours, or even a few days, you start to wonder when is a blinking “Cool on” light normal and when do you have to start troubleshooting. 

And if you have to troubleshoot, where do you start looking for possible causes? And if you’ve found a fault, how do you fix it?

When the “Cool on” light blinks on your Honeywell thermostat, it’s not necessarily a sign that there is something that needs fixing. When the light blinks only for a few minutes and then turns off or becomes a solid light, it means your air conditioning unit has started to work. But if the light is still blinking after about 5 minutes, it could mean that there is a problem somewhere in the system. Luckily, troubleshooting a blinking “Cool on” light is normally not a big issue.

In this article, we’ll look at the most common reasons why the “Cool on” light is flashing, determine whether it is “normal” or not, and provide tips on how to fix issues that could be the reason for the continuous blinking. 

Is a blinking “Cool on” light always an indication of a problem?

If your HVAC system is working correctly and the temperature in the house needs cooling, you should see the “Cool on” light blinking from time to time. Like the “Heat on” light, the thermostat informs you that it has started its work within your HVAC system. It’s turned on the air conditioning cooling unit in your home. 

This flashing light is a sign that your HVAC system’s safety measures to protect the compressor are in place. The “Cool on” light will blink when the HVAC signals the air conditioning unit to start and will blink for up to about 5 minutes while the system is working through all its elements and startup phases. The light will turn off or become solid as soon as the air conditioning works properly. If you have had a power outage, it might also blink.

Thus, when the “Cool on” light is blinking for less than 5 minutes from time to time it is not a signal of any trouble. Actually, you need to worry if the light is never flashing in the summer because that might mean that your air conditioning never works. 

How to fix a blinking “Cool on” light

If the “Cool on” light is blinking for longer than 5 minutes you most probably will also not feel cool air coming from the air conditioning unit.  Then you have to start troubleshooting. 

We have a few tips for you which will hopefully help you to solve the issue. If you follow the easy steps, you will in most instances find the problem and be able to fix it.

Lower the thermostat setting

Your first step is to determine whether your thermostat is able to control the cooling in your home. Lower the temperature setting to the lowest available setting, and leave it like that for at least two hours. Also, ensure that the thermostat is set to “Cool” mode and that the fan setting is on “Auto.”

After 2 or more hours, check if there has been a change in temperature in your house. If the house is cooler and the “Cool on” light blinks only for a few minutes when you set the temperature back again to the temperature you want the home to be,  the system has most probably just needed a cool-off period and is now functioning correctly again. 

If you don’t encounter the issue again, you don’t have to worry anymore – you’ve fixed it by letting the system cool down. 

Check the thermostat’s setup mode

Homeowners sometimes forget that a power outage could affect your thermostat. It is possible that the “Cool on” light will keep on blinking after a power outage.  If you’ve been on vacation and when you come back to your home you find that the “Cool on” light keeps on blinking, there most probably has been a power outage while you’ve been away. 

Fortunately, you can easily fix this issue. The reason the light is flashing is that the power outage has switched the thermostat back to setup mode. When a Honeywell thermostat is in setup mode it normally causes the “Cool on” light to blink. 

If the thermostat shows you that it is in setup mode you just have to set your thermostat again and the problem will be solved automatically. And while you’re setting the thermostat again, also ensure that the thermostat’s time settings are on and correct. Incorrect time settings can cause the “Cool on” light to blink. 

Check the thermostat’s batteries

Sometimes the “Cool on” light will blink when your thermostat can’t work efficiently because of low batteries. Have a look at your thermostat and check whether the “Batteries low” light is also flickering (if your thermostat works with batteries). 

Luckily, this is also easy to rectify. Just change the batteries in the thermostat and your problem will be fixed!

Something to remember is that it is good practice to frequently check whether the “Batteries low” light is not blinking. When it starts to blink, you normally have about 2 months to replace the batteries before the batteries totally fail.

Check the thermostat’s and HVAC’s wiring 

If you have a thermostat that doesn’t use batteries but runs on 24 VAC instead, you’ll have to check the wiring. To do that, you’ll have to take the thermostat from the wall and open it. 

Remember to cut the power to the thermostat and the HVAC system before you remove the thermostat from the wall to open it. This is one basic safety rule never to forget! 

To ensure that no power is coming to the thermostat, the best is to put off the circuit breaker controlling the HVAC system.  If you don’t know which circuit breaker is the correct one, turn them all off. 

Unscrew the thermostat from the wall and check the thermostat for the C-wire. Also, check all the wire connections and fasten all the small screws again. 

If you’re not entirely sure how to check the wiring, it is recommended that you contact your local electrician or HVAC technician. 

Check the power supply to all HVAC devices

If the “Cool on” light is still blinking after you’ve taken the previous steps, you’ll have to check all the HVAC devices and ensure that all their power connections are still intact and that they all receive power. Ensure that everything is plugged in correctly and the switches are all turned on. 

If you’re satisfied that everything has been checked and you are sure all the connections are still secure and the devices are all plugged in and the switches are turned on, put off all the circuit breakers, wait a few seconds, and then put them on again. It just might solve your problem!

Remember that before you put a circuit breaker on again you have to press it down completely until you hear a click sound, and then put it on.

If you still haven’t find the problem, check all the devices’ fuses and replace any blown fuse you find. 

Check the indoor filter and air conditioning unit’s coils

All your HVAC equipment and devices, including your air conditioning unit, might be working fine, but if your indoor filter is not clean and needs replacing it can affect everything in your HVAC system.

Ideally, you must change your air conditioning filter somewhere around every three months. If the filter is dirty or blocked your air conditioning unit has to work harder. This situation can also cause the “Cool on” light to flicker and cleaning or replacing the filter might solve the problem.

 A dirty filter could also affect the components of your thermostat and cause a drastic temperature change. This can contribute to a situation where the “Cool on” light can blink.

Just like the filter, the air conditioner’s coils can become dirty and even blocked and cause the “Cool on” light to flicker. To quickly see whether the coils need any cleaning, you can check your air conditioning unit’s fins. If they are dirty, you can be sure the coils will also be dirty.

For safety reasons, make sure you turn off the unit before cleaning the coils. After you’ve cleaned the coils, your blinking light issue might disappear automatically. 

Reset your thermostat

After you’ve checked that there are no problems anywhere in your HVAC system and the “Cool on” light is still blinking you can reset your thermostat back to the original factory settings. All your programming and preferred settings will be wiped clean. (Just remember to write down the settings before your reset – you will need them during the resetting! If you haven’t written them down you’ll have to waste time to work out all the programming again.) 

The way to reset a Honeywell thermostat varies from model to model. If your Honeywell thermostat is powered through the C-wire, you will have to make sure that you cut all the power to it before you do anything just to be on the safe side.

There are Honeywell models that come with a menu button that allows you to reset by simply pressing and holding this button for a few seconds. This action triggers a “Reset,” “Factory,” or “Factory reset” prompt. In some models, you’ll find this under the “Preferences” option of your thermostat.

The best is to look in your manual, or online, how to program your thermostat again.     

Last resort: Phone a professional HVAC professional or electrician

If the “cool on” light still keeps on blinking after you’ve reset and re-programmed your thermostat, you’ll have to call a professional HVAC technician or electrician to come and check the system for you.


A blinking light on your HVAC thermostat normally means that something is not working or has failed. Therefore it is normal to worry about a flickering “Cool on” light. Fortunately, this is the one light that does not always tell you about faults, but most of the time gives you peace of mind that your HVAC system is still working. And when the light is indicating that there is a fault in the system by blinking continuously, it is usually not a big issue to troubleshoot and rectify the problem.

We hope that this article with tips and guidelines for troubleshooting when the “Cool on” light is blinking will help you whenever you encounter this issue. 


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