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

Honeywell App

It’s a hot summer day, and you want to relax in the cool AC. You switch on your air conditioning and relax, waiting for the cool air to come through. But keep waiting. Perplexed, you check for what’s wrong and see your Honeywell Thermostat flashing “Cool On.” So how do you fix this issue and get the cool air back on?

Quick Answer

The easiest way to deal with this is to set the temperature to the lowest setting and reset your thermostat. Other methods include checking the AC coils, filters, and batteries. There are different methods of exploring each of them.

You can try each of these methods until you resolve the issue. Let’s see the steps for each of them in detail.

Ways To Fix a Honeywell Thermostat That’s Flashing “Cool On”

Here are seven ways you can fix a Honeywell thermostat that’s flashing “Cool On”.

Method #1: Switch the Temperature to the Lowest Degree

Use this method to check if your thermostat can regulate the temperature or not. Follow the steps below:

  1. Check if the cooling works by changing the controller settings.
  2. Set the fan setting to “Auto” while the mode is set to “Cool.”
  3. Now switch the temperature to the lowest setting possible.
  4. Keep it at the same setting for a few minutes and check if there is any noticeable change.

Method #2: Check if the Clock Isn’t Set After a Blackout or if the Thermostat Is in Setup Mode

If there has been a power outage or blackout at your place, this might work for you. The thermostat has probably shifted the thermostat to the setup mode.

Check if the thermostat is off or not set. This would cause the indicator to blink. If any of these things have happened, configure and review the settings as per the instructions.

Method #3: Check the Batteries

You may have to replace the batteries if they are weak. Once the thermostat displays a low battery, you have two months until it dies. If the batteries are drained fully, the thermostat won’t perform. Check the battery status on the thermostat display.


If your thermostat does not use batteries, check the wiring of the 24 VAC.

Method #4: See if the HVAC System Components Have Power

  1. If there is something wrong with the system components, you will hear humming or clicking noises.
  2. Check if the fans, furnace, air handler, or AC unit have power or not.
  3. Check if the connections are correct. See if the supplies and sockets are properly connected and switched on.
  4. Look out for any unscrewed components and see if the doors are properly closed.
  5. The unit must be working properly without any item blocking it.
  6. Also, turn the circuit breakers off and on by switching off the unit to ensure there is no error. Now, using a voltmeter, you can check the blown fuses.

Method #5: See if the AC Filter Needs Replacing

If the AC filter isn’t working properly, overall cooling will still be affected despite everything else being fine. You need to replace it every three months to prevent dust and dirt from clogging it.

If the filter clogs, the AC unit works harder to cool the surroundings. The compressor and other equipment pressurize because of this. You may even face a huge drop in temperature or problems with other HVAC components.

Method #6: See if the AC Coils Are Dirty

Similar to the AC filter, the AC coils may also get dirty. The external coils are dirty. Or maybe there is a blockage in the HVAC unit in this case. Dust gathers on the coil for years and eventually clogs it, preventing the airflow. The coil cannot absorb heat, and this affects the overall cooling.

Switch off the unit and clean the coils and the surrounding area too. This way, they won’t clog again in the future. Ensure the unit is in an open or wide room with no furniture or plants blocking it.

Method #7: Reset Your Thermostat

If nothing else works, this can be the last resort. Reset the thermostat to its factory settings. This erases all the previous data. The device also goes back to default settings.

  1. First, check the model of the thermostat.
  2. Now, write down the current configurations.
  3. Disconnect those powered by a C-wire.
  4. Press and hold the “Menu” button for a few seconds to reset it.
  5. Once reset, enter the previous configurations.

Finally, if nothing works, check the user manual or call customer support to help you out with the issue.


In this hot weather, a thermostat breakdown can be pretty frustrating. To fix it quickly, follow a few steps. Check if the thermostat is reset or if the HVAC components have any problems. If nothing works, reset your thermostat or call customer support for help.

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