Honeywell Thermostat Not Turning On AC – Solved! [2023]

Honeywell Thermostat is currently leading the market for producing the best thermostats for your heating and cooling systems.

But like any other electrical appliance, sometimes they require troubleshooting too.

According to the users, one of their most common issues was their Honeywell thermostat not turning on AC.

This is why I have broken down the most possible reasons for your Honeywell thermostat not working and how to take care of each one.

If you are in the same mess, keep reading!

Why is Your Honeywell Thermostat Not Turning On the AC?

Reason 1: Low Battery

Honeywell thermostats are smart devices that give you a low-battery warning by showing a ‘LOW BAT’ signal about 1-2 months before the batteries completely dry out.

But if you have been away from your home for some time, you may have missed the warning.

But you can still look for some symptoms that show your thermostat is running out of batteries.

Firstly, you will notice that the thermostat has become somewhat stubborn. This means it won’t respond properly the first time you try to turn on or off your AC.

That flat batteries are the reason for this unresponsiveness.

Secondly, imagine you are trying to set the most comforting temperature setting for your house, but the AC seems to be ignoring your thermostat’s command.

This is another red flag of low batteries.

When the thermostat’s batteries start drying, the AC starts to respond less to the temperature adjustments.

If you find yourself in this position, clearly your Honeywell thermostat is begging for a fresh pair of batteries.

Some Honeywell thermostats are powered by low-voltage wires rather than batteries.

If you have replaced the batteries already, yet the thermostat isn’t responding, you should call a qualified HVAC technician to look at the issue.


  • Before you begin, it’s important to turn off your heating or cooling system.
  • This prevents any mishaps during the battery change.
  • Carefully pop off the cover of your thermostat.
  • You might need to use the screwdriver to pry it open gently.
  • Inside the thermostat, you’ll see where the batteries are located.
  • They’re usually your regular-sized AA batteries and placed in a compartment near the top or bottom of the thermostat.
  • Take out the old batteries by simply lifting them out.
  • Put the new batteries in the same slots, matching the plus (+) and minus (-) signs as you saw with the old ones.
  • Carefully snap the thermostat cover back into place. Make sure it’s securely attached.
  • Reactivate the heating or cooling system and relax.

Reason 2: Wrong Settings

If you have installed a new Honeywell thermostat and haven’t quite figured out the correct settings yet, it may cause your AC not to turn on.

It’s like plugging your phone into your charger and forgetting to turn on the power outlet, a simple mistake we all have made at some point.

With the wrong temperature settings and mode, your AC is more likely to not respond to your thermostat when trying to turn it on.


  • Make sure the thermostat is in the correct mode for your needs.
  • If you want to cool your home, it should be set to ‘cool.’
  • For heating, it should be on ‘heat.’ If it’s too warm or too cold, set it to ‘auto.’
  • Check the desired temperature setting.
  • If the temperature is set to the exact room temperature, it won’t turn the AC on.
  • If the mode is set to ‘cool’ and the temperature is set higher than the room temperature, or the mode is set to ‘heat’ and the temperature is set lower than the room temperature, the thermostat won’t turn the AC on.
  • Look at the fan setting. You can usually set it to ‘auto’ or ‘on.’
  • ‘Auto’ means the fan runs only when heating or cooling is active. ‘On’ keeps the fan running continuously.
  • If your thermostat has a timer, ensure it isn’t turned on and blocking the thermostat’s command.

Reason 3: Bad Wiring

If the batteries are replaced and the thermostat is still not turning on the AC, there’s a high chance that it has some wiring issues.

If that’s the case, you will notice some strange behavior from your AC, apart from being unresponsive.

The AC won’t maintain the temperature set in the thermostat. And not just that, it will keep turning on and off by itself like it was possessed.

You will also hear strange clicking and buzzing noises from your AC terminals as it is about to emit electricity.

These are the clear symptoms that there is something seriously wrong with your AC unit’s wiring system.


  • Head outside your home and locate the control panel for your AC system.
  • It’s usually close to the outdoor unit of the AC system.
  • Take a look at the wiring connections inside the control panel.
  • Make sure all the wires are firmly connected and there are no visible signs of damage to the wires.
  • Now, check the wiring of your thermostat.
  • You’ll find wire connectors on the rear or side of the thermostat.
  • Confirm that these cables are correctly connected and don’t have any obvious damage.
  • Trace the thermostat wires from the thermostat to the control panel.
  • Ensure the cables are safe and free from apparent damage as you go along.
  • Inspect the terminals where the wires are connected.
  • There’s a red and a white wire, and they turn on the thermostat.
  • Make sure these wires are connected.
  • Now, move to the R and Rc terminals on your thermostat.
  • If the R terminal has a wire and the RC terminal is empty, use a jumper selector switch, a jumper plug, or a small jumper wire to connect R and Rc.
  • If the RC terminal has a wire, but R is empty, move the wire from RC to R and use a jumper, as mentioned above.
  • If both R and RC have wires (not jumpers), you don’t need to connect them.

Reason 4: Clogged Air Filters

The air filters in your AC block the dust and other air particles and improve your home’s inner air quality.

Over time, dust and debris accumulate on the air filters, restricting the AC’s air purifying process, which in turn causes the AC to shut down and not respond to the thermostat.

You will start noticing certain areas of your home are not cooling evenly. That’s when you know you have a clogged air filter at hand.

You may also notice a rattling or whistling sound coming out of your AC before it becomes completely unresponsive to the thermostat commands.


  • Begin by turning off your air conditioning system.
  • Air filters are typically installed near the return air duct or inside the indoor unit of your AC system.
  • Gently remove the clogged air filters from their housing.
  • They should slide or pop out without much effort.
  • Use a soft-bristle brush or a handheld vacuum cleaner with a brush attachment to remove loose dirt and dust from the filters gently.
  • If your filters are washable, rinse them under lukewarm water.
  • Use a mild detergent to clean them if needed gently.
  • Avoid using harsh chemicals or scrubbing vigorously.
  • After washing, allow the filters to air dry completely.
  • This may take a few hours, so be patient.
  • Once the filters are dry, slide or pop them back into their housing, ensuring they’re properly seated and secure.
  • But if it has been more than 3 months since you last replaced your air filter, it’s time to replace it with a new one.

Reason 5: Tripped Breakers

AC systems are equipped with circuit breakers to protect them in times of a power surge.

An HVAC system usually has two breakers, one outdoor air conditioner breaker and another indoor air handler breaker.

If one of these breakers trips, the AC will completely shut down and stop responding to the thermostat.

If you are trying to turn your AC on through the thermostat, and instead of powering up, it is fluctuating the voltage of your entire house, there’s a strong possibility that a tripped breaker is causing that.


  • Turn off your AC system before you do anything.
  • Locate your electrical panel or breaker box; it’s usually in the basement, garage, or utility room.
  • Inside your electrical panel, look for two switches—one for the inside unit, often labeled as air handler, furnace, or FAU (Forced air unit).
  • If the breaker switch is in the middle, it’s tripped.
  • To reset it, gently push the tripped breaker to the “off” and then flip it back to its original position.
  • You should feel it click into place.
  • For the outdoor unit breaker, search for a switch labeled “air conditioner.”
  • If you find that the outdoor unit breaker has tripped, reset it like the inside unit breaker.
  • Move it from the middle to “off” and then back to its original position.
  • To prevent future trips, make sure to clean or replace your AC’s air filters regularly and keep the condenser coils clean.

Reason 6: Blown Out Fuses

A blown-out fuse can often cause your AC not to turn on at the thermostat’s signal.

If you can’t find any visible defects in the thermostat and its wiring, the issue is more likely to be with the fuses.

Fuses can be found in the AC’s control board. When they are blown, the thermostat will still function properly, but it will not turn on or off the AC.

A blown fuse will cause a complete loss of power to the thermostat, making the display go completely blank.

You will also notice frequent breaker tripping if there are any blown fuses in the control board.


  • Locate the air handler compartment and open the cover of your AC’s control board.
  • Inside, look for small, cylinder-shaped fuses.
  • You’ll notice the blown fuse; it might look blackened or have a broken filament.
  • Carefully remove the blown fuse from its socket using a fuse puller or small pliers.
  • Be gentle; we don’t want to damage anything.
  • Get a replacement fuse of the same type and amperage rating.
  • It should be labeled on the metal caps of the fuse.
  • Insert the new fuse into the same socket where you removed the blown one.
  • Make sure it’s securely in place.
  • Keep an eye on the new fuse for a while.
  • If it blows again immediately, there might be a more significant issue.

Reason 7: Incompatible Thermostat

If your thermostat is relatively new compared to your HVAC system, there’s a high chance that the thermostat isn’t compatible with your AC.

As a result, the thermostat will function perfectly, but it won’t send any signal to the AC.

If you can’t find any technical issues with your thermostat mentioned above, it’s plausible that you have the wrong model installed.

Some newer models of Honeywell thermostats show an ‘error’ message on their display whenever they can’t communicate with your older HVAC system.


  • First, check the user manual or visit the Honeywell Thermostat’s official website to ensure your thermostat model is compatible with your AC system.
  • It’s important that the thermostat supports all the configurations of your AC system.
  • If you find that your thermostat is incompatible with your AC system, you need to replace it with a compatible model.
  • Choose a model that is designed to work seamlessly with your AC system type.

Reason 8: Faulty HVAC System

If none of the solutions seem to work, maybe your thermostat is functioning properly and there’s an issue with your AC system.

When your thermostat sends a signal to the capacitor, it gives the compressor the necessary power to turn on.

A starter relay sends this electrical charge from the capacitor to the compressor. If any of these two are damaged, the thermostat will click but won’t turn on the AC.

If this is the case, you may hear strange clicking noises from the outdoor unit. It is because the compressor tries to start without the required power from the capacitor.


Capacitors and relay breakers are sensitive electrical components. And without proper expertise, you should never attempt to repair or replace them on your own.

If you find the symptoms showing something wrong with one of these two, you should turn off your AC system entirely and contact a technician to take care of the issue.

Bottom Line

Though it’s good to consult a professional whenever you see your Honeywell thermostat not turning on the AC, we can all agree that paying for such a small issue is a hefty fee.

This is why whenever you fall into this pit, try narrowing the reason of what is causing the problem so that you can fix it following the instructions of this blog.

But if you notice any electrical sparks or burning smell coming out of the thermostat or the AC unit, shut them down immediately and call your nearest electrician immediately.


How do I reset my Honeywell thermostat that won’t turn on?

To reset your Honeywell thermostat:

  • Turn on the thermostat.
  • Press and hold the menu button for 5 seconds.
  • Scroll through the setup menu and select ‘reset.’

Can resetting thermostat fix AC?

Sometimes after a power surge or an outage, the AC blows warm air even though the thermostat mode is set to ‘cool.’ Resetting the thermostat in this condition can fix the AC as well.

What is mode on Honeywell thermostat?
The system modes on the Honeywell thermostat are Auto, Heat, Cool, and Emergency Heat.