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If “Cool On” is flashing on your Honeywell Thermostat, it can mean one of two things. Either the AC system is kickstarting which can take up to 5 minutes and is perfectly normal, or there’s a bigger problem which you need to troubleshoot. We help you with a step by step guide.

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At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

What does the “Cool On” indicator mean?

Let’s talk about Cool On really quick. The Cool On indicators is one of the system/operation modes regulated by your thermostat. Like Heat On, it simply informs you that it has started its work within your HVAC system. It’s turned on AC cooling in your home. 

If your Honeywell thermostat is flashing Cool On for a short period, it doesn’t mean there is anything wrong. It is a safety measure built into the system. It’s to protect your compressor from being damaged, giving it around 5 minutes to cycle its elements. This can also happen if power loss occurs.

9 steps to troubleshoot a blinking “Cool On” indicator

If the “Cool On” indicator blinks for much longer than 5 minutes and you do not feel cool air coming through your system, there you need to troubleshoot the issue. Follow the steps below to try and pinpoint the problem. 

1. Set Thermostat to Lowest Temperature Setting

Here you’ll check if the thermostat is able to control the cooling. Try adjusting the controller settings to see if it is giving a call for cooling.

Set it to the minimum temperature reading. Make sure the mode is setting to Cool, also that the fan setting is set to Auto. Leave it for a while and see if anything kicks in or if there are changes in the temperature.

2. Check if thermostat is in setup mode or if the clock is set

Did a power outage occur recently or while you were away? This can cause your thermostat to switch to setup mode. This can cause the Cool On indicator to blink. Check also the time on your thermostat if it’s off or not set. Review and configure your settings accordingly.

3. Check the thermostat batteries

If the thermostat batteries do not have enough power, it won’t be able to initiate cooling or heating. If so you need to change the thermostat batteries.

Check for a Battery Low indicator on the thermostat display. You usually have around 2 months of power once your thermostat starts displaying this.

If your thermostat runs on 24 VAC instead of batteries, the wiring needs to be checked. To do this, you’ll have to power down your system. Then you need to detach the thermostat and check for the C-wire. Heads up, some thermostats have to be unscrewed first.

Check out our guide on how to change Honeywell thermostat batteries

4. Check if Air Handler/Fans, Furnace, and AC unit have power

At this point, you might have to draw a bit more patience. If the first few diagnostic steps didn’t seem to improve anything, it’s time to move forward.

When checking your units, be aware of sounds such as unusual humming or clicking noises.

Check your equipment. Are they getting the power they need? Have a quick look at the power supplies, connectors, sockets, etc. Make sure they’re plugged properly and the switches turned on.

Check the components on the equipment. Are the doors closed properly? Are there loose or unscrewed parts? Is there stuff blocking or keeping the equipment from working properly?

Check also the circuit breakers for anything unusual. Try turning on and off your equipment and also at the circuit breaker. Further technical actions might have to be done like checking for blow fuses with a voltmeter.

5. Check indoor AC filter and see if it needs changing

You can have all your other equipment, including your AC unit, working fine. But if your indoor filter needs replacing, then the cooling in your home will be greatly affected. Aside from the air quality, your thermostat, equipment, and power bill will also be affected.

Ideally, your AC filter needs to be changed somewhere around three months. If the filter is clogged or dirty, it causes your AC unit to work harder and makes it more likely to break down sooner. It could also cause problems to its components and cause a high temperature drop. This could be because of blocked registers or a partially frozen indoor coil.

6. Check AC Coils and see if they are dirty

Just like your indoor AC filter, check also for dirty or blocked external AC coils. After months or years of operation, your coil will collect dirt that prevents proper airflow to it. It won’t be able to absorb heat and cool the air as well as before.

Check your AC unit’s coils and you’ll see if it needs cleaning if there’s buildup on the fins. Make sure you turn off the unit before cleaning the coil.

To further minimize clogging the AC sooner, clean also the area around it. Also, try to give it enough space from surrounding elements (such as plants or leaves) to have better airflow.

7. Reset your thermostat

After isolating possible problems from other equipment, you could reset your thermostat back to factory defaults. Your settings and program will be wiped clean so you may want to write down what configurations you might forget.

The steps for resetting Honeywell thermostats vary depending on the model. If it’s powered through the C-wire, you’ll want to power it down first to be safe.

If your model has the Menu button, press (or press and hold) the button for a few seconds and you’ll find either Reset, Factory, or Factory Reset. On some models, you can find it under the Preferences option. Check out this link for more instructions on resetting your thermostat to factory defaults. You’ll also find instructions for other models.

After resetting, set your thermostat again to your previous configurations or change it as desired.

8. Check Manual

Don’t forget your usual manual! It could have information to help with your concerns. Some of those might be specific to your thermostat’s model. The vague prompts on your thermostat display are not always helpful. But check out your manual for explanations on common hitches.

Lost your physical manual? You’ll likely find it somewhere on the web. We also have a handy list with most Honeywell thermostats’ manuals. There’s a search feature, and to easily identify your model, you can also check the images included.

9. Call in the professionals!

If you’ve already tried all of the above and nothing works, it may be time to call in the professionals. If your HVAC system has been performing properly recently and suddenly stopped working, it may be an easy fix for a pro.

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Examples of typical issues a trained technician can resolve include damaged wiring, a faulty capacitor, blocked sensor, or a blown fuse. Your system’s compressor, condenser, or fan motor might also need further checking.

If it’s a new installation, the wirework needs to be checked for correct matching. Also, your thermostat needs to be correctly configured to your system. Your experts can help you with that.


We hope this guide helps in explaining what “Cool On” means, when it is normal to see it and when it might point to a problem. 

The above steps should hopefully save you some time and money in getting you back up and running. Leave a comment if the above helped or if you have other tips to include in the troubleshooting list.


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