Nest Thermostat Not Turning on A/C: Causes and How to Fix! 

Google’s Nest Thermostat is becoming increasingly popular as more and more people outfit their homes with Smart devices that allow them to take agency over their home’s technology.

Nest Thermostats are designed with self-learning AI firmware that lets the thermostat change its heating or cooling behavior based on the temperature of the rooms it’s connected to rather than waiting for you to become so uncomfortable that you change the temperature of your own accord. For many people, the increased energy efficiency results in the device paying for itself via utility bill savings.

However, as with any technology, many bugs and conflicts can occur when using the Nest Thermostat.

One such issue many people run into is the Nest Thermostat’s inability to turn on their air conditioning units. This can be extremely frustrating during the summer months when not turning on your air conditioner at the right time can cause you to swelter while you wait for the air conditioning to kick in. 

Here are the most common causes of the Nest Thermostat failing to turn on an air conditioning unit and the steps you can take to try and fix it before you call a professional into your home.

Some Quick Troubleshooting Tips

Before we get too far into the weeds, it is important to correctly diagnose the nature of your thermostat problem.

If your Nest thermostat is unable to turn on the A/C immediately upon installation, incorrect wiring is extremely likely to be the cause of the problem. Other problems with a fresh installation could include incompatibility with an HVAC system or a faulty Nest, but these are far less likely.

If your Nest thermostat previously worked, but now fails to turn on the A/C, you need to promptly identify whether the issue is being caused by: (a) the Nest thermostat, or (b) something else in your HVAC system. The easiest way to make this distinction is to uninstall the Nest and reinstall your old, “not-smart” thermostat.

If the system runs the A/C properly with the old thermostat, then you know that the Nest is the cause of the failure. If the system still doesn’t work, even with the old thermostat, then you know that the problem is related to your HVAC rather than your Google device.

(1) Your Thermostat’s Energy History Is Causing a Conflict 

If your thermostat uses a lot of energy to heat or cool your home, it might cause your thermostat to act unexpectedly. Your thermostat will try to conserve energy to prevent it from causing you excessive bills; it’s part of its learning capabilities! But as with any learning machine, it can learn the wrong information if you don’t teach it correctly. 

Start by opening the Google Home app, then touch and hold your thermostat’s tile. This will bring up additional options, and you’ll want to select the Energy Dashboard option. Select Day, Week, or Month to see your device’s energy consumption history. 

You can also check your device’s energy consumption from the Nest app. Open up the Nest app and tap your Nest Thermostat from the application’s home screen. Then Tap “History” to bring up the Nest Thermostat’s energy history for the last ten days. Tap on any of the days to get more details about why the temperature changed during those days. 

Lastly, you can check your device’s energy history directly from the device. Open the Quick View menu on the Nest Learning Thermostat, then select “History” to see the energy consumption data the system has gathered. 

(2) Check for Installation Errors 

If your Nest Thermostat isn’t experiencing energy consumption issues, you may want to check its wiring to ensure the thermostat is wired correctly.

Faulty wiring can cause your Nest Thermostat to interact with your other devices, and even with itself, strangely. So, checking the wiring is a good place to start if you’re unsure whether your Nest Thermostat is working as intended. 

To check for installation errors, start by turning off power to the Nest system in the fuse box or system switch. Opening up a device while it’s still on and has power is a recipe for disaster with electric appliances. So your best bet is to kill power to the device entirely. 

Once you’re sure your Nest Thermostat has no power, pull the display off the Nest Thermostat to reveal the wires behind it. Ensure that all the Nest Thermostat connectors have associated wires and, if needed, reinsert wires to ensure that the connector button stays fully pressed. If the connector button isn’t pressed down, your Nest Thermostat won’t be able to register a wire connection. 

If needed, you may need to re-strip and straighten the wire ends so they can be properly seated into the Nest base’s connectors.  

(3) Get Your System Professionally Maintenanced 

Nest Thermostats do need professional maintenance from time to time, much like a car. If your system has issues, it may require professional care to get it back on track and functioning again. Try restarting your Nest Thermostat to see if the problem is remedied but returns after a while of use. 

Turn the power to your Nest Thermostat off at the breaker switch in your fuse box. Wait a few minutes for the system to pull down, then turn the power back on. If your system functions correctly at first, but the fault returns after the device has been running for some time, it needs to be professionally serviced. 

You can diagnose possible system maintenance issues by disconnecting the Nest thermostat and reconnecting your old thermostat. If your system works correctly, there may be an issue with your wiring or power source. You may need a standard wire or Nest Power Connector to use the Nest Thermostat.

Your plan may need maintenance if your system doesn’t work correctly after reinstalling the old thermostat.   

(4) Check that the System Is Compatible With Your Power Source 

Nest Thermostats are compatible with most standard 24V systems. However, there are a handful of designs and models that are incompatible.

Others may need a C wire installation or a Nest Power Connector to be used. Check Google’s official list of systems and hardware that aren’t compatible with Nest thermostats to ensure that your system is compatible with Nest’s hardware. 

(5) Fix or Connect a C Wire 

Pull the thermostat’s display off to check if there’s a wire in the C connector. If there’s a wire in the C connector, ensure that all cables are connected correctly and securely.

If the display shows an error code or wiring report, consult Google’s support database of error codes.  

If there’s no wire in the C connector, check if there’s a C wire connected to the system’s board that you can connect to your thermostat. Be sure to turn off the power to your entire home at the breaker, fuse box, or system switch to prevent damage to your system. 

Look at which wires are connected to your system’s control board and see if any of them are connected to the system’s C terminal. If there’s a wire connected to the system’s C terminal, you can put the other end of that wire into your Nest Thermostat’s C connecter. 

If there isn’t a wire in the system board’s C terminal, you’ll need a Nest Power Connector or to have a C wire installed into the system board. 

It’s not recommended to install a C wire yourself. Google recommends getting an HVAC maintenance professional to install a C wire into your system as this can be risky to both your system and you. 

If you need to have a C wire installed, there are a few options for handling this. One is to have a Nest Power Connector installed instead. This device provides power to your Nest thermostat without needing a C wire. It’s typically easier to install and less expensive than running a new wire into your system. 

(6) Verify that Battery is Sufficient

Although the battery life on a Google Nest is impressive, it still drains eventually. When in doubt, check the obvious things (like power sources, batteries, and WiFi connection) before assuming that your device has a rare or hard-to-fix problem.

(7) Verify a Strong WiFi connection

Similar to the above point #6, this may seem obvious.

But many smart device issues stem from a weak or unreliable WiFi connection. Just because you previously had a strong signal doesn’t mean that you can’t lose WiFi coverage over time.

(8) Regardless of the Culprit, Try Restarting and/or Resetting Your Nest

Even if you aren’t able to pinpoint the problem, resetting and/or restarting the Nest thermostat may be the only thing needed to fix the thermostat.

Google’s support page does a great job of explaining the steps involved in restarting and resetting a Nest. Remember that a factory (“hard”) reset will lose all of your device settings, so only try this if you have exhausted other options.

(9) Verify Thermostat Settings

Before we sign off, we should discuss one last troubleshooting step that will seem obvious to most readers.

Many thermostat “failures” are actually caused by incompatible settings.

For example, a thermostat won’t trigger an A/C to kick on unless it is set to Cooling mode. Sometimes in our frustration, even the tech savvy among us can make a simple mistake like this.

Look through your settings and make sure that you don’t have anything programmed that is actually causing your Nest thermostat to “incorrectly” keep the A/C from coming on. For example, maybe you have Cooling mode on but your set temperature is 85 degrees. Or you have it programmed to only run the air conditioning during a certain window of time.

Again, sorry for stating the obvious. Our intention isn’t to insult your intelligence, but rather to exhaust all troubleshooting resources before giving up and calling tech support.

(10) Flip the Breaker for Your A/C

In all likelihood, your air conditioner is on a different breaker than your Nest thermostat. It is entirely possible that you are getting power to the Nest thermostat, but no power is being supplied to the A/C.

In this case, you just need to flip the breaker and wait for your HVAC to turn on.

Final Thoughts 

It can be frustrating when your Smart Home technology malfunctions, but luckily it’s pretty easy to get your Nest thermostat working correctly.

When it comes to servicing Smart Home technology in this way, we recommend having a professional assist you with the maintenance. Poor or improper technique with this level of electrical wiring can have disastrous consequences.

Google has prepared a list of “Local Pros” that partner with Google to provide maintenance services to Nest products. Check that list to get a professional diagnosis and installation.

If you are tech savvy and knowledgeable with electrical work, you should try rewiring and resolving the problem yourself. Otherwise, just call the professionals.

Thankfully, most Nest thermostat problems that occur out of the blue aren’t related to incorrect wiring, but instead caused by an easily fixable issue like a software update, device reset, or settings adjustment.