How to fix a Honeywell thermostat that’s flashing “Cool on”

How To Fix A Honeywell Thermostat That’s Flashing “Cool On”

Normally when the “cool on” light starts flashing on your Honeywell thermostat, it’s nothing to worry about. This usually means that the air conditioning unit is starting to get to work.

If you leave it for a couple of minutes, the light should switch off when the AC starts to work properly. 

However, if the light doesn’t turn off after 5 minutes, this could mean that there’s a bigger problem at hand. It can be confusing trying to work out how to fix the issue – especially if you don’t have the manual to hand!

We’re going to walk you through a few steps that you can take to troubleshoot your problem.

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Why is my Honeywell thermostat flashing “cool on”?

So before we cover how to sort a Honeywell thermostat that’s constantly flashing “cool on”, we need to know more about how it works. The “cool on” light is one of the operation modes that your thermostat controls.

Similar to the “heat on” setting, “cool on” is just telling you that the thermostat is starting up to cool your home.

You should be used to seeing your thermostat flashing “cool on” for short periods of time. This doesn’t necessarily mean that anything is wrong with your thermostat. This is a safety measure that comes with the thermostat to protect the compressor.

The “cool on” feature allows the system to work through its elements so that it can do its job properly. If you have a power cut, it can also flash then.

How to troubleshoot the “cool on” light

Don’t feel any cool air coming through your system? Has the “cool on” light been flashing for much longer than 5 minutes? Then we’ve got a few solutions for you to try which will hopefully help you to solve the issue.

Lower thermostat setting

One of the easiest fixes is to test whether the thermostat is able to control the cooling in your home. Lower it down to the lowest setting, and leave it for an hour or two to see if the temperature changes at all. It could be as simple as the system just needs to cool down.

Make sure that your Honeywell thermostat is set to “cool” mode, and that the fan setting is on “auto”. Set the thermostat to the lowest temperature reading possible.

Once it’s been left to do its thing, check to see if anything has kicked in or if there’s a change in temperature in your home.

Is the clock set or is the thermostat in setup mode?

A factor that can cause your Honeywell thermostat to flash “cool on” repetitively is when a power cut has happened. If you’ve come back from a vacation and noticed that the thermostat is flashing “cool on” repeatedly, it could be an indication that this has happened.

There’s no need to panic! All this means for your thermostat is that it’s been switched back to setup mode. This can cause the “cool on” light to blink repeatedly at you. Once you’ve set up your thermostat again, this problem should solve itself. 

It’s also worth checking the thermostat’s time settings to ensure that this isn’t turned off, or isn’t showing as set. This could also be a factor preventing your Honeywell thermostat from working effectively.

Look at the thermostat batteries

Sometimes the thermostat won’t be able to work properly if its batteries don’t have enough power. It won’t be able to heat or cool your home if the batteries don’t have any juice in them! Luckily this is an easy enough thing to fix.

All you need to do is change the batteries in the thermostat.

Take a look at the display to see if the “battery low” light is flashing. As soon as your thermostat starts to display this warning, you normally have around 2 months before the power will start to fail.

So if this is a flashing light you’ve noticed before you saw the “cool on” light flickering, this could be the issue.

Have a thermostat that runs on 24 VAC instead of batteries? Then you’ll need to get the wiring checked. You must ensure to do so safely as otherwise you could electrocute yourself. Remember to fully power down the system first.

When there’s no power going to the thermostat, you can detach it from your wall. You will need to unscrew it from the wall to do this. 

Check the thermostat for the C-wire to make sure that everything is in working order. If you’re not entirely sure how to check the wiring, it will be best to phone an electrician to help you.

Do the AC unit, furnace, and air fans have power?

After following the above steps and noticing that the “cool on” light is still flashing after a long time, it will be crucial to take a deep breath and try to be as patient as possible. We know it can be a frustrating process trying to work out what exactly has gone wrong.

Each time you check your Honeywell thermostat, be sure to note if there are any strange humming or clicking noises. This could give an indication that something else is wrong.

Make sure to give your thermostat equipment a thorough looking over. Is everything getting the right amount of power that it needs to work properly? Look at any connectors, power supplies, and sockets.

Is everything plugged in correctly and the switches are all turned on?

Look at all the components on your Honeywell thermostat equipment. Do all the doors close properly? Is there any part that’s become unscrewed or loose? Does it look like there is anything blocking the equipment or preventing it from doing its job?

Next, take a look at the circuit breakers. Does anything appear unusual? Turn the equipment off and on again. Try also doing this at the circuit breaker.

If you still can’t work out what exactly is going wrong, you may need to take more technical actions with a voltmeter to see whether any fuses have blown.

Does the indoor AC filter need changing?

If your indoor filter needs replacing, this can throw the whole system out of whack. Even if everything else about your Honeywell thermostat equipment is working just fine!

An indoor filter that needs replacing can affect everything from your air quality to your thermostat and its equipment. It can even affect your power bill!

It’s important to remember that your AC filter will need to be switched every three months or so. When your AC filter becomes dirty or clogged, it can affect the performance of your air conditioning unit.

The unit will have to work harder, which can make the filter break down quicker.

 A dirty filter could also affect the components of your thermostat and cause a drastic temperature change. This can happen when the indoor coil becomes partially frozen or if it blocks thermostat registers.

Are the AC coils dirty?

Another component you can check is the AC coils. You should check to see if these have become blocked or dirty just like the filter. After a couple of years, dirt can build up on the coils. This will prevent airflow from properly getting to it.

A dirty or blocked coil won’t be able to cool the air or absorb any heat as well as it has done in the past.

You should know from looking at the AC coils if these are the issue or not. There will be a buildup of dirt on the fins. Before you clean your AC unit, it’s important to practice basic safety and switch it off.

If you want to prevent the AC from getting clogged up quicker, make sure to clean the surrounding area. You will also need to allow enough space between the AC unit and anything else you have in your home which could affect its airflow.

This will include things like plants or leaves.

Give your thermostat a reset

Now that you’ve made sure there are no problems with all the other aspects of your thermostat, you can reset it back to its original factory settings. The program and preferred settings that you have installed will be wiped clean, so remember to note these down before you do so.

You wouldn’t want to forget them!

Resetting your Honeywell thermostat will vary between different models. If your particular thermostat is powered through the C-wire, you will need to ensure that you power it down before you do anything just to be on the safe side.

Models that come with a menu button can be reset by simply pressing and holding this button for several seconds. This should then trigger a “reset, factory, or factory reset” prompt. You may find this under the “preferences” option of your thermostat if it’s a different model.

Once you’ve reset your thermostat to the factory settings, you can then select your preferences.

Look at the manual

One of the most obvious solutions of course is to check the manual of your particular thermostat. This should have some information to help you work out what exactly has gone wrong if you can’t quite work it out. There may even be specific tips for your particular Honeywell model.

Don’t have your manual anymore? As long as you know the model number of your thermostat, you should be able to find it on the Honeywell website or by getting in touch with their customer services.

Phone an engineer

Sometimes you can give everything a go and still not be able to work out what the issue is. In this case, it will be time to give a professional engineer a ring. If your HVAC system has been working perfectly fine up until now, it could be an easy fix for them to sort for you.

A professional engineer will be able to sort out any damaged or faulty wiring, a blown fuse, a blocked sensor, or a faulty capacitor. They can also check to see if your HVAC system’s fan motor, condenser, or compressor needs more attention.

Newer installations could have a fix as simple as making sure the wires are all in the right place. A professional engineer can also help you to configure the system properly with your thermostat.

In summary

We hope you’ve now been able to sort your issue of why the “cool on” light is flashing on your Honeywell thermostat! You should now know what this light means exactly, and how you can fix it if it’s been flashing for longer than 5 minutes. Hopefully you’ll save money and get it running again in no time at all!

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