How to Fix Honeywell Thermostat Cool on Blinking?

Has your Honeywell Thermostat’s Cool On light started to blink? Don’t worry. It is a far more common problem than you may think. In fact, in 90% of cases, it isn’t really going to be a massive problem. It is just a ‘feature’ of the Honeywell Thermostat. The only time that you really need to worry is if it is happening frequently.

So go through this entire guide to know how to fix the Honeywell Thermostat ‘Cool On’ blinking.

Why is the Honeywell Thermostat ‘Cool On’ Blinking?

As we just said, in the vast majority of cases there isn’t really going to be an issue here. Most of the time, that light is going to be blinking because your Honeywell Thermostat temporarily lost power.

The light blinking is really indicating that one of the safety features in the unit is working. It is trying to ensure that the compressor in your system has time to cycle before it is kicked back into action. This basically means that you don’t end up killing the AC compressor, which can be rather costly to replace.

If this is a one-off and the light is not blinking for more than five minutes, then it is fine. You have nothing to worry about. There isn’t a problem to fix.

However, if you see that the problem continues to happen, or if that light just never steps blinking, you do have a bit of troubleshooting to do.

Check the Circuit Breakers

As we said, in most cases, the Honeywell Thermostat is going to be blinking if there is a power cut. Therefore, we suggest that you go and check the circuit breakers for your HVAC system. You need to make sure that nothing has tripped.

If something has tripped, and it does continue to happen, then you will probably want to talk to an HVAC technician as this could indicate that there is something wrong with your system. Although, thankfully, that it is going to be quite rare.

Restart the Entire System

If you have waited five minutes and the light hasn’t stopped blinking, then we suggest that you turn everything off in your system.

Leave it off for a couple of minutes and then switch it back on again. This sometimes triggers a proper reset of the system and clears the issue. Most people find that the problem is properly rectified at this point. 

Check the Batteries on your Honeywell Thermostat

In some cases, the light flashing means that the Thermostat is not able to send a cooling signal to the AC. While there could be a multitude of different reasons for this, chances are that it is going to be something as simple as the batteries in your Thermostat needing to be replaced. This is a quick job.

If you aren’t quite sure how to tackle changing the battery, then just pull out the manual for your Thermostat. This will guide you through the process. Each Honeywell Thermostat is different.

Remember, there are some Honeywell Thermostats that only have a battery as backup power. The main power will come from your mains supply. If that is the case for you, then you can skip this step. 

Switch to Thermostat to the Lowest Possible Temperature 

Some people have reported that by switching their thermostat to the lowest possible temperature, it has been enough to kick the system into action.

You may still need to wait a further five minutes for that light to switch off. However, eventually, you may start to feel the temperature change. Once you do, the ‘Cool On’ light will stop blinking.

Clean AC Filters

If you haven’t had your AC looked at in a while, then you may want to clean the AC filters. If they are blocked, then they may be causing the AC to not function correctly. This, ultimately, is going to result in the Honeywell Thermostat struggling to send that cooling signal to the AC. This will leave the light blinking.

Don’t worry if you have never cleaned your AC filters before. It is a rather simple job. The process will vary from AC to AC, so you will need to consult that manual.

If you haven’t done anything to your AC filters in over a year then we suggest that you look into replacing them rather than cleaning them.

Clean AC Coils

The same goes for your AC coils. These are a bit trickier to clean, but it is something that you will be able to do yourself. Consult the manual for your AC if you are unsure how to clean your coils.

This is a job that you need to carry out at least once per year! It should be part of your yearly maintenance routine with an HVAC technician.

Reset the Honeywell Thermostat 

If you have gone through the previous steps and that light is still blinking, then you have one more thing to try i.e reset your Honeywell Thermostat. This is quite a convoluted process, hence why we placed it at the end. The process will vary from thermostat to thermostat so, as always, you do need to be consulting that manual.

To make the process a little bit easier for you, make sure that you note down any settings before you reset the system. It will make it a whole lot quicker for you to get everything back up and running.

Consult an HVAC Technician 

If you have gone through all those steps and the light is still continually flashing, then there isn’t much more that you can do.

Your only hope is to contact an HVAC technician to see whether they are able to identify the problem. in some cases, it may be an issue with the power supply, or there could be a problem somewhere in the AC that you simply cannot fix yourself.


As you can see, learning how to fix a Honeywell thermostat ‘cool on’ blinking isn’t that difficult. Most of the time, you just need to wait about five minutes.

If that doesn’t fix it, then there are plenty of troubleshooting steps you can do on your own. The problem will be fixed somewhere along the way. It is exceedingly rare that you will need to get in touch with an expert to rectify the problem. 


This is the site where I share all of the information about home security. I have been working in this industry from last 10 years and therefore, sharing all my experiences and learnings.

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