Honeywell Thermostat Won’t Turn On AC? Fix it In Minutes

A thermostat is a vital piece of HVAC equipment and can have issues that users may find hard to troubleshoot on their own. Honeywell is one of the most popular thermostat manufacturers in the market, and if your Honeywell thermostat won’t turn on ac – then you are at the right place. In this article, we will cover the causes behind this common problem and its quick yet efficient solutions.

Common Causes behind Honeywell Thermostat Not Turning On Ac with Solutions

1. Weak Thermostat Batteries

One of the major reasons behind the Honeywell thermostat not turning on ac is weak batteries. On batteries running out, the thermostat won’t have a constant power supply and you might experience interruptions. Generally, batteries can stay up to a year without replacement, but even rust and dust can cause a bad connection in the battery holder.

Most thermostats have batteries and are powered by HVAC systems, but some run on batteries only. If your Honeywell thermostat uses batteries, it won’t be able to turn on the AC in this case. You can check the status of your thermostat battery on the screen. The pop-up message of “No Power Source” implies the presence of a damaged battery. If you see this message on your screen, it is time to change your thermostat’s battery. 


Fresh batteries inserted into a clean battery holder may quickly solve your issue. The next time you decide to clean the batteries or replace them, make sure that you are done with the process as quickly as possible. If you take too long to insert the fresh batteries, your Honeywell thermostat might reset, and then you will have to reprogram all your settings.

2. Bad Thermostat Wiring

Another reason behind your Honeywell ac not turning on can be bad wiring. Each wire going to your thermostat has its functionality such as heating or cooling. Incorrect thermostat wiring or loose connections could be the reason for your Honeywell thermostat not turning on your air conditioner. 


Open your thermostat and ensure that all wires are connected to their correct terminals. There is no standardization on the color code because of which you should check the wiring designation on the control panel of your furnace.

Another great way of fixing this issue is to go through the user manual of your HVAC system. For ac functionality, the R and RC terminal needs to be connected separately.

3. Tripped Breakers

HVAC systems are installed with circuit breakers that are necessary to protect the devices in times of a short circuit. If a tripped circuit breaker cuts the power to your air conditioner, it cannot be turned on by your thermostat, and if the power to your thermostat is cut, your air conditioner will not receive any message from the thermostat. 


Resetting a breaker should solve your problem here. Turn on all the tripped HVAC and thermostat-related switches in your home’s circuit breaker box. If the breaker switch is in the middle, then you will know that it is tripped. When you turn on a circuit breaker, remember to press the switch down completely and then press it up until you hear a clicking sound. Turn on the circuit breaker again and switch on the thermostat.

However, if the breaker trips again immediately, there could be another problem with the system or one of the other devices. At this point, you should call HVAC technicians to have a look at the system.

4. Misconfigured Thermostat

If you’ve upgraded your HVAC system or your thermostat, your Honeywell ac won’t turn on as it may no longer be compatible with your thermostat settings.


Get your user manual and check out how to properly reconfigure your thermostat model. Make sure it is correctly configured for your system and all its stages. If you can’t figure out what your settings should be, contact an HVAC professional for advice.

5. The thermostat is not configured to cooling mode.

Honeywell thermostats turn on AC automatically only when the cooling mode has already been activated, or else an error appears. To determine this, check if its screen displays “COOL ON”.


If the “Cool On” message is not displaying on your thermostat’s screen, you have to enable cooling mode on the thermostat.

Honeywell thermostats turn on the feature of cooling automatically only in one condition – the room temperature is higher than the desired (and entered) temperature. To test whether your thermostat and air conditioner are communicating, set the temperature much lower than room temperature. The thermostat should then turn on the air conditioner.

Frequently Asked Questions

Q1 Why is my thermostat not turning on my AC?

Ans: There are many reasons for thermostats not turning on ACs, but the main ones can be that the thermostat can be bad, or there is a problem with the wiring or electric component in your system.

Q2 How do I turn on the AC on my Honeywell thermostat?

Ans: Locate the switch on your Honeywell thermostat and choose the option to cool.

Q3 Why is my Honeywell thermostat on cool but not working?

Ans: There can be several reasons as to why your Honeywell thermostat is on cool but not working, such as bad wiring, drained batteries, the desired temperature higher than that of the room, and so on.


When your Honeywell thermostat won’t turn on ac, try out the various troubleshooting tips mentioned above. Fortunately, most issues are relatively easy to pinpoint and fix, which homeowners can rectify themselves. 

Finally, if nothing seems to be helping, perhaps it’s time to consider replacing your thermostat. Before buying, just make sure your new thermostat is compatible with your HVAC system. If the problems persist, contact Honeywell support for further assistance with your specific model.

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