Honeywell Thermostat Flashing Cool On After Changing Batteries: the Reasons and Solutions

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If you find your Honeywell thermostat flashing cool on after changing batteries, then don’t panic yet because this is actually common issue of the devices. It may mean nothing if it happens for a short time only, but it may require more attention if the flashing occurs for more than five minutes. Here are several troubleshooting tips you can perform on it.

Troubleshooting Steps for Flashing Honeywell Thermostat

Below are some alternatives steps you to take when you find your Honeywell thermostat flashing cool on after changing batteries:

  • See if the thermostat’s clock is set or it’s in setup mode

Did your household experience power outage as of recent or while you weren’t at home? If so, your thermostat may go into setup mode that can result in blinking Cool on indicator. You may also check and reconfigure the clock settings to make it set accordingly.

  • Inspect the indoor AC filter

In theory, it is recommended to change the AC filter approximately once every three months. A dirty or clogged filter will make any unit to function harder, which may result in faster break down. Moreover, this may result in sudden drop of temperature or other component issues.


  • Inspect the AC coils

Similar to the AC filter, you will also need to examine the external AC coils to check if it is blocked or in dirty condition. It is especially important for old unit that has been working for extended time, as dirt and dust may be collected by the coil, preventing air to properly flow. Your thermostat won’t be able to set the right temperature if this issue occurs.


You also need to examine if the coils need some cleaning by looking at the buildup of dirt. Remember to turn it off before you do some cleaning work. Furthermore, to lower the possibility of clogged up unit, it is also recommended to clean the nearby area.


  • Try to reset the thermostat

If you have tried all the tips above and yet to find a success, then you may opt to reset the thermostat. This will cause the device to be back into the default settings. Other programs and installed settings will be removed, so note down any configuration you’ve made before.


To reset the thermostat, you need to know what exact Honeywell model that you own. For instance, a C-wire powered thermostat need the electricity connection to be turned off first before getting a reset for safety measure. On the contrary, a device with panel button features only needs you to press a button and hold it for several seconds to perform a factory reset. Other models may need you to execute the reset through the menu of ‘Preferences’.


In this case, you need to open the manual instruction of your specific thermostat model. After the reset is done, then set the configurations according to your preferences.

Most of the time, the issue of Honeywell thermostat flashing cool on after changing batteries will be resolved by using the tips above. However, if none works, then you may need to call a professional technician to check if your device went under more serious problems such as blown fuse, blocked sensor, or damaged wiring.