When you discover that your Honeywell thermostat is not turning on your home’s Air Conditioner, you immediately wonder what is causing the problem and also whether you can troubleshoot it yourself. 

The reasons why your Honeywell thermostat is not turning on your AC are varied and include aspects such as depleted thermostat batteries, tripped circuit breakers, bad wiring, configuring or compatibility issues, and in some instances, a thermostat that needs a replacement. However, you can troubleshoot and fix most issues yourself.   

In this article, I’ll discuss different reasons why a Honeywell thermostat might fail to turn on the AC and then how you can determine the reason why your home’s AC is not turned on by your Honeywell thermostat. I’ll also suggest ways to fix the problem.

A honeywell thermostat on a wall

1. Tripped Circuit Breakers

This is a possible problem that most DIY-ers can usually troubleshoot and fix themselves

The possible problem

 HVAC systems, and thus your thermostat as well, are installed with circuit breakers. This is necessary to protect the devices when there is a power surge. Sometimes the whole system is served by only one circuit breaker, but often there are separate breakers for devices such as your air conditioner. 

If, on the one hand, a tripped circuit breaker cuts the power to your air conditioner it cannot be turned on by your thermostat, and on the other hand, if the power to your thermostat is cut, your air conditioner will not receive any message from the “dead” thermostat. 

How to fix the problem

Check all the switches in your home’s circuit breaker box. Turn on all the tripped HVAC and thermostat-related switches. You will know that a breaker is tripped if the switch is in the middle position. When you turn on a circuit breaker, remember you have to press the switch down completely and then press it up until you hear a click sound. When the circuit breaker is turned on again, switch on the thermostat. Your problem might be solved.

Something to keep in mind

If, however, a circuit breaker immediately trips again there is most probably another problem in the system or one of the devices.  If you are not a qualified electrician yourself, it is recommended that you rather call your HVAC technician to come and check out and repair the system.

2. Weak Batteries

This is also a possible problem that most DIY-ers can usually troubleshoot and fix themselves.

The possible problem

If your Honeywell thermostat uses batteries, it won’t be able to turn on the AC if the batteries have become weak or when a bad connection has developed between the batteries and the battery holder’s terminals. The batteries may be old, or rust and dust could’ve, over time caused a bad connection in the battery holder. Fresh batteries inserted into a clean battery holder may solve the problem, and it is quick and easy to replace batteries.

Check for a “no power source” message on the thermostat’s screen

Most Honeywell thermostats’ screens display the message “No power source” when the batteries or battery connections are faulty. If you see this message, weak batteries are in almost all instances the cause of your problem.

After you’ve replaced the batteries with fresh ones, your thermostat should turn on the AC again. If it is still not turning on the AC, you’ll have to troubleshoot further. 

Future situations like this can be avoided by regularly checking for messages on your thermostat. Most Honeywell thermostats display a “Low Bat” warning message about 1-2 months before your batteries will be completely depleted. Replace the batteries when you receive the low-battery message, and there will always be enough battery power enabling the thermostat to turn on the air conditioner.

Good to replace batteries in any case

But even without the message on the screen, the batteries could still be the problem. Sometimes they are too weak to enable your thermostat to turn on your air conditioner but not so weak that they trigger the thermostat to display the error message. Because it is so easy and quick to replace the batteries I recommend that you always replace the batteries with fresh ones as part of your troubleshooting procedure if they’ve been in the thermostat already for more than 1 year. 

How to replace the batteries

To get access to a Honeywell thermostat’s batteries you have to remove the thermostat from the wall panel.   

Before you remove the thermostat, turn off the thermostat, and then turn off the circuit breaker(s) servicing your thermostat and HVAC system. In most cases, it is only one circuit breaker. If the circuit breakers are not labeled, be safe and turn all of them off to ensure that no current is reaching the thermostat. 

Most Honeywell thermostats just slide out of the wall mounting. Remove your thermostat from the wall, and open the battery holder on the side of your thermostat. Remove the old batteries and clean the inside of the holder. Insert the fresh set of batteries and close the battery holder. 

Remount the Honeywell thermostat on the wall again. Ensure that it clicks into position. Turn on the circuit breaker(s). Remember, to turn on a circuit breaker, you have to press the switch down completely and then press it up until you hear a click sound. When the circuit breaker is turned on again, switch on the thermostat.    

Something to keep in mind

When you replace batteries and/or clean the battery holder’s terminals, you should do it quickly. If you take too long to insert the fresh batteries, your Honeywell thermostat might reset, and then you’ll have to reprogram all your settings. 

3. Thermostat not set to “Cooling Mode”

This is another possible problem that most DIY-ers can usually troubleshoot and fix themselves.

The possible problem

Your Honeywell thermostat can be set to a specific mode. If it is not in the “Cooling Mode” your thermostat will not turn on the air conditioner.  It will only be able to switch on the air conditioner if the cooling mode is activated.

How to fix the problem  

You know your thermostat is in the cooling mode if the Honeywell thermostat’s screen displays “Cool On”. If the message is not displaying, you have to enable cooling mode on the thermostat. Look for the slider switch labeled “Heat, Off, Cool” on your thermostat and slide the switch to the “Cool” position. To set the temperature to your desired temperature, use the arrows on the screen.

Something to keep in mind  

The thermostat will only turn on the air conditioning if the set temperature is lower than the temperature in your home. In other words, if you’ve set the temperature in the “Cool Mode” on 70 degrees, but it is 69 degrees in your home, the air conditioner will not be turned on. To test whether your thermostat and air conditioner are communicating, set the temperature much lower than 69 degrees. The thermostat should then turn on the air conditioner. 

4. Your furnace is off

This problem can usually be fixed by homeowners themselves.

It may sound strange that you have to ensure that your furnace is on when you are worried about the AC which is not turning on.

Possible problem

In many homes, the HVAC blower motor is shared by the air conditioner and the furnace. Thus, if the furnace is off the air conditioner will not work. Because the blower motor is located within the furnace, cool air could stay in your air conditioner if your furnace is not on. 

How to fix the problem

Ensure that the furnace’s switch and the circuit breaker are on and that the door of the furnace is securely closed.  If the air conditioner starts to work, you’ve fixed the problem

5. Misconfigured Thermostat

This is a possible problem that DIY-ers with HVAC experience can usually fix but often need help from an HVAC technician.

The possible problem

If you’ve upgraded either your HVAC system or your thermostat, it might be that the thermostat is not configured correctly. 

How to fix the problem

For full details on how to configure your Honeywell thermostat correctly for your HVAC system, you’ll need to consult your thermostat’s manual or find a DIY video online which is specifically aimed at your Honeywell model. In short, the basic steps are as follows:

  • Ensure that you have your thermostat’s “Date Code.” If you don’t have it, you can find it at the back of your thermostat. Write it down for later use.
  • Tap on the Menu option on your thermostat’s touchscreen and select “Installer.”
  • When you’re prompted to enter your “Date Code” enter it and your installation settings will be displayed.
  • Your manual or the DIY video will explain how you have to select the functions and settings for your specific HVAC system. 

If you can’t figure out what your settings should be, contact your HVAC professional for advice.  

6.  Bad Thermostat Wiring

This is also a possible problem that DIY-ers with HVAC experience can usually fix but often need help from an HVAC technician.

The possible problem

Incorrect thermostat wiring or loose connections might be the reason why your Honeywell thermostat is not turning on your air conditioner. To check and repair the wiring you’ll have to take the thermostat off the wall mounting. 

How to fix the problem

Before you remove the thermostat, ensure that the thermostat and its circuit breaker are turned off. If the circuit breakers are not labeled, be safe and turn all of them off to ensure that no current is reaching the thermostat. 

When the power is off, just slide the Honeywell thermostat out of the wall mounting. You can now reach all the wire connections.

Your Honeywell thermostat has an Rc (Red cooling) wire which powers the cooling system in your home. To function correctly, the Rc wire has to be plugged into the Rc terminal on your thermostat. If it is disconnected, reconnect it. 

Also, check all the other wires and secure them all. 

Put your Honeywell thermostat back on the wall. Ensure that it clicks into position. Turn on the circuit breaker(s). Remember, to turn the switch down completely and then press it up until you hear a click sound. When the circuit breaker is turned on again, switch on the thermostat.   Your problem might be solved. 

Last resort – reset your Honeywell thermostat

If you couldn’t solve the problem with my troubleshooting tips above, resetting your thermostat might be a solution.

The two main reasons why I see it as a last resort are that in the first place, the problem might not be a thermostat issue as such, and secondly, you might have to reprogram your thermostat after a reset. 

Usually, resetting procedures differ from model to model, and therefore if you want to reset your thermostat as a last resort, check your manual or a DIY video to determine how to reset your Honeywell thermostat model. You find your thermostat’s Model ID at the back of your thermostat. So, you’ll have to remove the thermostat from the wall to get to it.  The ID is the numbers that follow the letters RHT, T, TL, CT, RLV or TH. 

Conclusion

There are various reasons why your Honeywell thermostat doesn’t turn on the AC, but, fortunately, they are relatively easy to pinpoint and fix. In most instances, homeowners can troubleshoot and rectify the issues themselves.  

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