Honeywell Thermostat Flashing “Cool On” [7 Easy Fixes]

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Is your Honeywell thermostat flashing cool on and don’t know what to do?  This is a common issue, especially in Honeywell pro series thermostats. There are a few reasons why your thermostat could be blinking ‘cool on’ on display. These include recovery from a power outage, low thermostat battery, clogged air filters, and a few others we’ll be looking at shortly in this article.

thermostat flashing "cool on"
Woman adjusting thermostat

Why A Honeywell Thermostat Would Be Blinking Cool On

When you experience a “cool on” message flashing on your thermostat, it means that your thermostat is in delay mode. The thermostat goes in delay mode to protect your heating and cooling system from damage. This usually happens after a recent power outage to your HVAC system. So this can occur when your system is recovering from a power interruption.

Because of that, it is completely normal to have the “cool on” flash on your thermostat’s display for a few minutes only. Typically, it takes about 5 minutes.

Read also: Furnace will not turn on with thermostat

6 Steps to Troubleshoot a Honeywell Thermostat Flashing “Cool On” Message

If the cool on message keeps flashing for more than 5 minutes, it could mean there is another issue with your HVAC system, and you can start troubleshooting the issue by following these tips:

1. Check Thermostat Settings

You first want to check your thermostat settings when you can’t feel any cool air coming through after 5 minutes have passed.

Usually, if there was a recent power outage to your HVAC system, your thermostat will automatically run into ‘set up mode.’ Set-up mode could be the reason for the cool on blinking message on your thermostat. To clear that flashing message, you need to make sure that:

  • Your thermostat has the correct setting. If you discover that your thermostat has gone into setup mode, give it the correct settings. You can go through your Honeywell thermostat user manual to do this.
  • Your thermostat has the correct clock settings. If the clock setting is wrong, set it accordingly. Giving your thermostat the correct settings will resolve most of these issues.

2. Check Your Thermostat’s Battery

When your thermostat’s batteries are dying, they usually bring issues and can negatively affect the normal operation of your thermostat. Common issues caused by low or dying batteries include:

  • Difficulties in turning on and off your HVAC system. The batteries don’t have to be dead completely to negatively affect how your thermostat operates.
  • Low batteries can also cause your thermostat to flash the “cool on” message.
  • A dying battery could cause your heating and cooling equipment not to respond to cooling and heating changes on your thermostat.

If you notice that you have low batteries, change them. Usually, 2 months before your batteries dying out, you will see a ‘low battery‘ warning message on your thermostat.

3. Set Your Thermostat to The Lowest Temperature

After having checked your batteries and they seem all Ok. Then find out if your system can respond to temperature settings on your thermostat.

You can do this by setting your thermostat to the lowest temperature and waiting for about 45- 60 minutes and see if you will start receiving the cool air.

While your thermostat is on the lowest setting, ensure it is also set to ‘cool mode ‘and your fan set to ‘auto’.

4. Check If Both the Outdoor and Indoor Units Have Power Supply

If the problem is not resolved at step # 3, then head over to check if there is power supply to your HVAC system. That is, both the outdoor and the indoor unit.

Find out if there is power supply to the components of your heating equipment. Because HVAC systems are high-wattage equipment, it is not uncommon for a circuit breaker to trip on any of its components. You may need to look out for blown-out fuses in a few cases. You’ll also need to pay attention to:

  • Any unusual sounds: Humming or clicking noises usually indicate an issue with your system.
  • Doors not closed properly: most doors of your HVAC system have automatic switches that automatically cut power to your system when not properly closed.

5. Clogged Air Filters Could Cause Honeywell Thermostat Blinking Cool On

You’ll be surprised to know that all other components of your HVAC system could be working fine but the problem remains with dirty air filters.

Dirty air filters cause most issues with your heating and cooling systems. When you have clogged air filters, your HVAC system might not be able to cool your home efficiently. Because of that, your heat pump or furnace will work harder and result in hefty energy bills.

Most manufacturers recommend changing air filters quarterly but if you have pets or air that has lots of dust, changing them more frequently is even better.

6. Dirty Coils Can Cause Honeywell Thermostat Flashing “Cool On”

After years or months of work, dirt and debris will eventually accumulate on the condenser or evaporator coils preventing the coils from absorbing or disposing of the heat feely.

Dirty or blocked coils will negatively affect how your heat pump system cools and warms your home. That is why it is always to inspect the coils and clean them if there is a build-up on them.

As a rule of thumb, clean the surroundings to prevent your outdoor unit from attracting dirt quickly.

7. Try Resetting Your Thermostat

After trying out all these possible troubleshooting steps and the problem does not disappear, you still have one ‘easy to fix’ option. That is, to reset your thermostat.

However, resetting your thermostat will erase all your settings and will restore the factory default settings. In this case, you may want to take note or save your settings before resetting your thermostat.

Depending on the model, the method of resetting Honeywell thermostats does vary. Still, you will typically press and hold the menu button for about 5 seconds and then follow the instructions that follow.


thermostat blinking "cool on"
Young lady adjusting thermostat

Why would my Honeywell thermostat go blank?

If you are using a honey well thermostat and you experience a blank display, this could mean that your furnace door is open or not properly closed.  It could also indicate a low battery.
Furnace doors have door switches that will cut off power to the system when the door is either open or not closed properly. You may want to double-check the door to rule out this issue.

How do you manually reset a thermostat?

Resetting a thermostat varies depending on the model. You can try resetting a thermostat manually by inserting batteries backwards for 5 seconds. You can also try pushing in a pin on the reset button. If your thermostat relies on main power supply, you can try tripping the circuit breaker to the thermostat.

How do I know if my Honeywell thermostat takes batteries?

Usually, Honeywell thermostats that use batteries are those with LED display. You can find out if your thermostat takes batteries by checking if your thermostat has a common or C-wire. If it doesn’t have, it probably takes batteries. The best way to know if your thermostat takes battery is to check your manual.

Can a Honeywell thermostat work without batteries?

Some thermostats designed to use power from main power supply do not need batteries. However, those that rely on batteries for power will not work without batteries.

Final Thought

If can be frustrating to discover your thermostat blinking cool on but does not do its job when needed. Regular and proper maintenance should prevent most of these unforeseen issues.

 We hope that whenever you are dealing with Honeywell thermostat flashing cool on, you will find this article helpful in troubleshooting that issue. So that you can have the problem sorted out before calling in HVAC professionals, which could save you some bucks.

Generally speaking, homeowners should have maintenance tune up for their heating and cooling equipment at least twice a year.


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