Honeywell Thermostat Blinking "Cool On"? (Here's Why!)

Benjamin Wright
by Benjamin Wright
It can be confusing when your Honeywell thermostat keeps flashing “Cool On”. There are several reasons that your thermostat displays this message, but it often indicates a problem with the AC. Follow along as we explore the possible causes and solutions for your Honeywell thermostat displaying a “Cool On” message.

Honeywell thermostats are among the best on the market because of their user-friendly display and reliability. The display is one of the highlights of Honeywell thermostats because useful information is readily available and it’s easy to use. So, what does it mean when your Honeywell thermostat is blinking “Cool On”?

The “Cool On” message will appear on your Honeywell thermostat if the batteries are dying or there is faulty wiring. Your Honeywell thermostat can also display “Cool On” if your AC needs to give your compressor a 5 minute cooldown time to protect it. A bad blower motor or dirty air filter can also cause a blinking “Cool On” to appear on your Honeywell thermostat.

Luckily, each of these problems has a quick solution ranging from waiting 5 minutes to replacing your AC filter. There is nothing to worry about if your Honeywell thermostat reads “Cool On”. Follow along as we explore what you can do if your Honeywell thermostat is flashing “Cool On”.

Related Content: How To Reset Honeywell Thermostats: A Guide For All Models | Is Your Honeywell Thermostat Blinking A Snowflake? | How To Change A Honeywell Thermostat Battery

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Possible Causes

Usually, the “Cool On” message on your digital thermostats should be stable. But when it flashes, it means the air-conditioner is engaged, or there is a technical malfunction. Here are a few of the things that can cause your digital Honeywell Thermostat to blink “Cool On”:

  • A power outage: A power outage or any other interruption to the power source can force the compressor to restart. This will initially put some pressure on the compressor and make the Cool On to blink for a while.
  • A compressor restart within a short period: If you manually readjust the temperature just after the HVAC system reached the set temperature, this will force the compressor to restart.
  • Switching of AC system from cooling to heating, or vice versa: Doing this will put a lot of pressure on the compressor and make your thermostat display blink.
  • Replacement of batteries: If you have just replaced your batteries, the Cool On display may flash on your thermostat for a few minutes.
  • Low battery: Sometimes, a Cool On flash is a thermostat’s way of telling you it is short on battery. While some Honeywell thermostats have a separate low battery notification, the older models still communicate with flashing signals.

Video: Fix for Thermostat Cool On Flashing

Honeywell Thermostat Cool On But Not Working

It is common for a “Cool On” message to appear and prevent your HVAC system from working for a few moments. All that you need to do is wait it out if your Honeywell thermostat says “Cool On” and won’t let you adjust the system. A static “Cool On” message simply means that your compressor needs 5 minutes to protect it from damage before you can program the thermostat again.


Simply wait 5 minutes and adjust your thermostat to see if it starts to work again. Your Honeywell thermostat won’t allow you to control the HVAC system during this period, and you shouldn’t try to override it. Contact an HVAC professional if the message still appears on your Honeywell thermostat after waiting 5 minutes.

Low Batteries

A blinking “Cool On” message can appear due to dying batteries, a power surge, or a power outage. Thermostat batteries last for an average of 10 months, so it is easy to tell when dead batteries are the culprit. Sometimes, a Honeywell thermostat can bounce back after a power outage without requiring a reset.

Power surges and power outages can disrupt your HVAC system unless you reset it in most cases. Your Honeywell thermostat may flash “Cool On” even when it’s brand new if it wasn’t installed properly. Luckily, the solutions for each of these problems are largely the same.


The easiest solution is to replace old batteries with new and compatible batteries right away if they are dead. However, you will need to reset the circuit breaker if you experience a power surge or power outage. You may have to shut off power to your Honeywell Thermostat for up to 30 minutes to reset your HVAC system and Honeywell thermostat after a power outage in some cases.

Dirty Air Filter

Every air conditioner has an air filter, and it can accumulate dirt and debris at different rates. However, an AC filter can gunk up quickly during periods of heavy use during the winter or summer. Dirty and clogged air filters can cause your Honeywell thermostat to display a “Cool On” message.

A dirty AC filter can cause several problems, such as thermostat malfunctions, high electricity bills, and frozen evaporator coils. An unexplainable “Cool On” message from your Honeywell thermostat can often indicate it’s time to replace the filter and prevent expensive bills and even mold or mildew.


HVAC professionals recommend that you replace your AC filter once per month or two months so that your system works properly. Otherwise, a dirty AC filter can cause your Honeywell thermostat to malfunction, read “Cool On” when it shouldn’t, or temporarily stop working.

You can also clean your AC filter with a mixture of equal parts water and white vinegar to fix your HVAC system and Honeywell thermostat.

You can spend as little as $10 to replace an AC filter yourself, or at least $40 to hire a professional. Replacing your own AC filter is easy and only takes a moment so it is worth saving the money.

Faulty Wiring

Faulty wiring can make a Honeywell thermostat malfunction and even read “Cool On”. Problems with wiring can occur even after installation if something gets knocked loose. However, wiring problems are most common for homeowners that wire their own thermostats and AC units.

That is why it is best for HVAC professionals to wire a thermostat to your AC, but DIY installations don’t always go wrong. Wiring problems can be dangerous, but they can also cause your thermostat to enter a “Cool On” or delay period without explanation. If your air filter is fine and you didn’t have a recent power outage or surge, the “Cool On” message could be due to faulty wiring.


It is best to hire an HVAC professional to rewire your Honeywell thermostat. However, that isn’t always an option, and it is possible to rewire it yourself. There are a few simple steps to rewire your thermostat, and that includes:

  • Shut off power
  • Disconnect and label wires
  • Keep track of color codes for wires
  • Connect new wires from the control board to the AC unit

Step 3 is the most important because you need to know the color codes for wires that go from your control board to the AC unit. Misaligning wires with the wrong color is the first mistake many DIYers make when wiring their thermostat. Hire a professional if rewiring your thermostat to your AC unit doesn’t work so that you don’t cause further damage.

Bad Blower Motor

Your AC blower is responsible for the airflow in your home, and it can negatively impact your HVAC system if it goes bad. The most common problem with AC blowers is that the motor goes bad. An AC blower motor lasts for an average of 12 years, but it can go bad before then.

Your Honeywell thermostat will likely malfunction if there is a problem with the blower motor. That is because your thermostat tries to turn the blowers on with no success. A blinking or static “Cool On” message can appear if your Honeywell thermostat unsuccessfully tries to turn on the blowers.


In this case, the only solution is to hire professional help. Working with air conditioner blowers and motors is difficult and can ruin your system if you make a mistake. HVAC professionals are qualified to carefully and efficiently repair or replace a blower motor, and it costs an average of $450.

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Related Questions

Why is the auxiliary pump of my thermostat coming on while displaying cool?

It can be quite confusing to have your home blowing hot when you set the temperature to low. There are a few possible reasons your auxiliary heat is coming on during the AC mode. But the issue is most likely with your wiring. The aux heat wire might be wired with the heat pump, thereby confusing the heaters, and making them come on in cool mode.A quick check on the terminal wiring is the right step forward. If the two are both on a terminal, move the aux wire to another terminal. Make sure the aux heat wire is in the W1 terminal, and the other is on W2.

Do I need to reset my thermostat settings if there is a power outage?

The Honeywell thermostat is designed with permanent memory storage, which helps store its program, schedule, and customizations. Every user setup is also stored. Therefore, there is no need for a reset after a power outage. When power gets restored, your thermostat will automatically pick up where it left off.

Summing It Up

The most common problem is that your compressor needs 5 minutes to cool down if your Honeywell thermostat is blinking “Cool On”. Otherwise, you may need to clean or replace your AC filter if it is clogged with debris. Simply reset your circuit breaker if your Honeywell thermostat says “Cool On” but won’t work after a power outage.

Inspect the wiring from your thermostat to your AC if no other solution works. Hire an HVAC professional if you have a problem with your blower fan motor and need to replace it.

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Benjamin Wright
Benjamin Wright

Benjamin is a proud homeowner who loves to write about DIY projects and home improvement projects. Traveling, perfecting his home, and spending time with his family are just a few of the many things that keep him inspired.

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