Smart Thermostat Troubleshooting Guide: Here's How I Fixed Mine

CNET

17-07-2025

In early July, when temperatures were rising to the 90s, my smart thermostat, a Nest Gen-4 model, shut down completely in the worst timing possible. None of the quick fixes I had used when testing previous thermostats worked this time, but things were getting uncomfortable, so I had to take the next step and contact a heating/cooling expert, which proved to be not just helpful but illuminating.
Along the way, I learned a lot more about my HVAC system than I knew and uncovered some very interesting sources of potential thermostat issues, even when it looks like nothing is working at all. That's also about the time I realized I was long overdue to offer a guide on what to do when your thermostat malfunctions right when you really need it.
In most cases, you can get your AC or heating back under control or at least know when to ask for outside help. Here's what to try for smart thermostats from Nest, Ecobee, Amazon, Honeywell Home and other brands.
Read more: 8 Reasons Your Smart Home Device Isn't Working and How to Fix It
Reboot your thermostat and check your Wi-Fi
Rebooting your thermostat and app is a helpful step in getting things working again.
Tyler Lacoma/CNET
I started with the basics for my nonresponsive thermostat, and you should, too. For me, that meant disconnecting the faceplate and seeing if I could reboot it or get it to respond at all -- a step that eventually required me to plug it into a charger to give its small battery some juice. I also rebooted my Google Home app to see if it could provide any information.
While my Wi-Fi connection wasn't the problem, this is also the time to check if your thermostat is connecting to the internet and if you need to input new Wi-Fi information, such as a new band or password. Connectivity issues may be responsible for many different issues.
Review your heating and cooling modes
Always double-check your thermostat mode. It may have been switched without you knowing.
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Thermostat modes can sometimes change without warning, and unless you lock your thermostat to app controls only, someone may have inadvertently messed with how it operates. That leads to common issues like a thermostat that's set to heating instead of cooling, one that's been switched away from an Auto mode, or perhaps accidentally put into a Hold mode so it can't change.
Thermostats may have different modes available depending on your heating and cooling system design, so run a broad check of your current status to make sure nothing obvious is wrong.
Check your error messages
If your thermostat shows an error code, look it up pronto.
Nest
With no power coming to my thermostat, it couldn't immediately show any error codes. However, the app was still functional and provided an "E" error code that I could look up to learn that the thermostat couldn't sense a power connection at all (not surprising, but a good step to check).
If your thermostat face or app displays any kind of error message, always look up the codes they provide. Those codes may offer insights into what's happening, and if you do have to contact a repair person, you'll have extra info to pass on to them. Many codes provide info on wires that may be required or placed in the wrong contacts, so it's a handy reference when setting up your thermostat, too.
Check your thermostat battery and power status
My thermostat has an LED light indicator to show whether the power is on, but there are other ways of telling, too.
Tyler Lacoma/CNET
Smart thermostats typically have a power source, such as a C-wire connection and a backup battery they can use when other power sources aren't working. If your thermostat is nonresponsive, you need to check power settings to see if it's getting power.
For my Nest thermostat, this was very easy, even if the faceplate wasn't getting power. The Nest Learning Thermostat 4 has a small LED indicator on the wiring mount itself that shows the power status, showing different colors if it's getting power and/or charging. My indicator light was out entirely, which confirmed the error code: No power was getting to the thermostat at all, which is why the battery had already died.
Not all thermostats have this option, but you can still make important power checks like visiting your breaker box to see if a heating/cooling breaker has flipped, or entering a Test Mode to see if there are certain units the thermostat isn't connecting to.
Examine your thermostat wiring
Wiring guides can help you check if there are any issues with wires.
Google Nest
With the faceplate removed, it's easy to check your thermostat's wiring, which should include at least several different-colored wires connected to different contact points. In a perfect world, these wires are labeled with letters and numbers so you can determine whether they're connected to the matching spots, but those labels can sometimes be hard to read. Wire color is another indicator, although this can also vary a bit.
As well as the right connections, see if it looks like any of the wires have come loose or aren't fully making contact. This is usually a problem you can fix yourself, but only touch the wires after you've shut down power to your HVAC system. Many thermostat apps have a little wiring guide in their settings that shows what the wiring contacts should look like, based on the system information you've entered. You can always call an expert if you don't want to handle this part yourself.
Check your app settings for wiring configurations
Dig into the settings and make sure your thermostat matches your cooling or heating setup too.
Tyler Lacoma/CNET
This step depends on the type of system and thermostat you have, but it's a particularly important one. It often allows you to find out why your thermostat suddenly stopped working, why it's blowing hot air instead of cold air, and so forth.
For example, I have a Rheem heat pump. Heat pumps generally need configurations set to Y, O, or B wires to properly switch between heating and cooling. My heat pump uses an O/B configuration, and this brand requires using the B wire configuration for cooling. My AC expert made sure to double-check this detail, but it's very easy to check on your own. Sometimes, wire configurations can automatically reset to the wrong wires (like after a blackout), and you need to go into your map and set them to perform correctly again. This is more common with heat pumps, but it can happen with other sources of cooling as well.
Now is also an excellent time to check similar settings to make sure the thermostat is configured to the proper type of system you have (electric, gas, forced air, radiant heating, etc.). Settings like these tell your thermostat how to control your system properly, and if they're wrong, then your heating and cooling won't work at all, or at least won't work as directed.
Take a look at your AC lines
AC drainage and clogs can cause thermostat issues, too.
Yusuke Ide via Getty
If your AC keeps shutting off and your thermostat looks like it's always rebooting, the problem could be more mechanical than digital. Check your air conditioner and look for any signs of leaking or puddles of water. This can indicate that an AC line designed to flush out accumulated moisture has clogged. This often triggers an emergency shutdown in the system, which is what can cause your thermostat issues as well. That means it's time for a thorough AC and drainage cleaning.
Check your GFCI outlets
If safety features tripped, your thermostat may have lost power as a result.
skhoward via Getty
My thermostat and power seemed to be working fine, so the expert I called had to step outside the box to find out what was happening -- and the answer was something everyone with a smart thermostat should know.
Ground fault circuit interrupter outlets, aka a GFCI, protect homes by automatically shutting off a circuit if they sense an electrical problem. An outlet in my garage, where my inside fan/cooling unit is, had triggered and shut down the electricity. The AC unit detected that emergency shutdown and stayed turned on, shutting down power to my thermostat.
Resetting the GFCI outlet restored power, to my relief, but we still had to find out what caused the power problem in the first place. My helpful technician found it appeared to be the power adapter for my irrigation system, which overheated or possibly had a power surge from a summer thunderstorm, causing a bit of scorching and triggering the shutdown.
With the mystery solved (I'll tackle irrigation technology another day), I was left with the reminder to check failsafes like GFCI outlets and other electrical protection your home may have. Sometimes, the issue can start in an unrelated spot.
Final notes
Remember, if you can't figure out what's going on or don't have a lot of experience with wiring, you can also contact customer support for your thermostat brand online, or call a heating/cooling service in your area to get some help. As I learned, the problem isn't always the thermostat, but sometimes a wider issue in your home's AC and more.
Now that you're schooled in thermostats, check out my home security cheat sheet, the best spots to install your security cameras, and tips on buying a smart lock for your home.