When your Nest thermostat suddenly stops turning on your air conditioning (AC), there are various things to consider when you are troubleshooting. However, many homeowners don’t know what the most common causes of such an issue are or what are all the aspects to consider when looking for possible reasons why the AC cannot be turned on by your thermostat.  

The most likely reason for your Nest thermostat not turning on the A/C is that the thermostat doesn’t get enough power or has a low battery. Loose wires or incorrect connections can also cause the problem.  Then there could also be issues with your Wi-Fi router. In some instances, the Nest thermostat and your air conditioning are just not compatible.

In this article, we’ll discuss the most common issues why the Nest thermostat doesn’t turn the AC on. We’ll recommend actions you can take in your troubleshooting process and offer possible solutions for the issues.  

We are not recommending a specific order, as it depends on your circumstances what you want to tackle first. Start with the aspects that can be checked and resolved the easiest in your specific situation.   

Check your Nest thermostat battery

If the battery is low the Nest thermostat may not be able to switch on your AC. The battery should provide about 3.6 volts to work properly. 

A thermostat’s battery can over time deteriorate and as it gradually reaches the end of its lifespan, the quantity of voltage it offers is too low to regulate your AC 

To rectify this, you have to charge the battery or replace it with a new one.

Here is a detailed guide on what to do if the AC is not working after a battery change.

Check your Nest thermostat wires

The Nest thermostat has several wires within its body. For the thermostat to work properly, every wire has to be inserted correctly into its terminal. Some of these wires might have loosened over time. If the thermostat has never put on the air conditioner since installation, the wires might have been inserted incorrectly.

For AC activation, the most important wire to be without any fault is the RC (Red Cooling) wire. This wire might be loosened, burned or inserted in the wrong terminal. 

To check the wiring, terminals and connections, you have to remove the thermostat from the wall for inspection – especially the RC wire. Ensure that the wires are inserted correctly and not damaged. The RC terminal has to be connected to activate the cooling function of your HVAC system.

If your thermostat has no RC or RH terminals but only one terminal marked with an “R,” the system’s R wire should be connected to the R terminal. This R terminal is the 24-volt hot terminal. The power originates at the 24-volt transformer and supplies power to the thermostat and then passes power to the devices, like your AC. 

Apart from the R, RC and RH terminals which you have to check on the Nest thermostat, there are other terminals as well to check.  The other terminals that commonly cause issues with your AC are the G terminal which controls the indoor blower fan relay, the Y terminal controlling the cooling relay in the condensing unit of your air conditioner, and the C-terminal which is providing power to the thermostat. 

Thus, when you inspect the wiring and connections, check all the thermostat’s terminals.

Clean your Nest thermostat

Your AC might not be turned on if your Nest thermostat is too dusty as dust often reduces the efficiency of a thermostat. The dusty environment in your thermostat also requires more power to execute its actions. This can cause the electric supply to be not sufficient for your thermostat to activate and operate your AC.   

This issue can easily be resolved. Remove the thermostat’s faceplate and remove the dust gently with a soft brush or canned air. 

Check the position of your Wi-Fi router

The Nest thermostat requires an internet connection to control your appliances such as your AC. If the internet connection is broken the thermostat will not be able to turn on your AC.

Frequent loss of the internet connection could be because your thermostat is too far away from your Wi-Fi router. 

This problem can be resolved by putting your router closer to the thermostat. As soon as the Nest thermostat is connected to the internet again, it will again switch on your AC when needed.  

Set your HVAC system on auto

If your HVAC is running non-stop or too frequently it might be the reason why your AC is not turned on by the Nest thermostat. When your HVAC runs continuously, it can cause the thermostat to read the room temperature incorrectly. 

Thus, if you expect your thermostat to power up your AC, you must not keep your HVAC on throughout the day. Set all the systems in your HVAC to “auto.” This should include your fan, AC, heater, and boiler. 

With every system turned to “auto” your HVAC will only come on when the temperature in your home rises or falls below your desired temperature. Your Nest thermostat will then put the AC on when needed.

Check the thermostat’s circuit breaker

Sometimes, something can cause your Nest thermostat’s circuit breaker to trip. You may not be aware of this and think the thermostat is still on. So, check the circuit breakers and ensure that you put the thermostat’s breaker on if it is tripped and check whether it stays on. 

Remember when a circuit breaker has tripped you have to click it completely off before you put it on again. If you put your thermostat’s breaker on again but it trips again there is an electric fault somewhere in your HVAC system or thermostat. You’ll most probably need a technician to assist you to find the problem.

Check compatibility

Although Nest thermostats are normally compatible with most air conditioners, there is the possibility that your AC is not compatible with the thermostat. Nest thermostats are sometimes not compatible with high voltage systems. 

Contact your Nest distributor and the manufacturer of the air conditioner to find out whether you might have a compatibility issue. If it is a compatible problem you’ll have to decide how you are going to fix it. You might have to replace either the air conditioner or the thermostat. 

Reboot your Nest thermostat

To function effectively a Nest thermostat needs not less than 3.6 volts of power. Determine the thermostat’s voltage quantity and if it is too low a reboot might be necessary.

Determine voltage quantity

To determine the voltage quantity 

  • click on “Settings” on the thermostat’s display,
  • tap on “Device Information,”
  • select “Power,” and
  • choose “Battery Label.” You’ll find the voltage quantity.


If the voltage quantity is below 3.6 volts, reboot the thermostat by taking the following steps:

  • Press the Nest thermostat’s ring and hold for about 10 seconds.
  • When the screen goes blank, release the ring.
  • Switch on your thermostat by pressing the ring on the thermostat again. 
  • Release the thermostat’s ring when the screen turns on. 
  • The Nest logo will appear, indicating that the thermostat is restarting.

You have now rebooted your thermostat.

Activate cooling mode on Nest app

Before you now switch on your AC from your Nest thermostat, you must first activate cooling mode on your Nest app. This is done as follows: 

  • Open the Nest app on your mobile device.
  • Click on the Nest thermostat button. (The button is the one that displays the thermostat’s current temperature.)
  • Click on “Heat” on the screen.
  • You now get a pop-up with a list of options.
  • To set your thermostat in cooling mode, choose “Cool” in the pop-up list.
  • Reduce the temperature to your desired temperature by moving the cursor on the temperature tab.
  • Exit your Nest app.

Switch on AC from the thermostat

Now you can switch on your air conditioner from your thermostat by executing the following actions: 

  • Go to the main menu on your thermostat by pressing the thermostat’s ring.
  • Turn the wheel on the ring to the thermostat icon.
  • Select the icon by pushing on the icon.
  • You’ll see the same list of options that you’ve seen on the app. 
  • Turn the wheel again to select “Cool.”

Your air conditioning should start working now.


As indicated in this article, there are various reasons why your Nest thermostat doesn’t turn on your air conditioner. For your convenience, we’ll summarize the actions you can take in troubleshooting. 

Remember, they are not in priority order and you can execute the actions in any order. Your circumstances will to a great extent dictate the order you are going to work.

We will, however, recommend that you consider rebooting the thermostat as an action early in the troubleshooting process. Like most digital devices, rebooting is sometimes all that is needed to rectify an issue.  

The actions you can take in the troubleshooting process are:

Check the wires, terminals, battery, circuit breaker, the position of the Wi-Fi router, and compatibility. Remove dust from the thermostat. Set your HVAC on “Auto.” Reboot the thermostat 

We sincerely hope that this article has given you a broader insight into the possible aspects that can be the cause of your AC problem. As indicated, the problem is most probably not with the air conditioner as such, but a thermostat or connection issue.

If your troubleshooting couldn’t fix the problem, you’ll have to consult a professional technician. 


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