Nest Thermostat Not Working: Power, Eco and Resets

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the maker’s manual

Nest Thermostat Not Working: Power, Eco and Resets

Short answer

Most Nest faults are one thing wearing three disguises. Without a C wire the thermostat borrows power through the heating and cooling wires, which drains the battery, restarts the screen and makes the system click on by itself. Everything else is usually Eco mode deciding the house is empty, or the few minute delay before the system is allowed to start.

A Nest thermostat not working is usually a power problem wearing a disguise. A screen that goes dark, a thermostat that restarts itself, a system that clicks on for a second with nobody asking, and a battery that never charges are all the same fault.

Another common one is not a fault at all. It is the thermostat deciding the house is empty and dropping the temperature on purpose.

Before you start

What to carry: a small flat screwdriver and a USB cable. Ten minutes.

Which one have you got? The round one with a metal ring is the Learning Thermostat, and it runs off an internal rechargeable battery kept topped up by the system. The flatter mirrored one is the cheaper Nest Thermostat, and it takes two AAA batteries as well.

Where the power comes from. On a gas furnace there is a switch on the side of the cabinet and a breaker behind it. On an electric air handler it is the breaker alone. That is the 24 volt supply the thermostat lives on, so a dead Nest is quite often a furnace that has lost power rather than a thermostat that has failed.

Switch the system off before the display comes off its base. Twenty four volts will not hurt you, but touching two thermostat wires together blows a small fuse on the furnace control board and kills the whole system. It is a cheap fuse and an hour of confusion.

Power off at the furnace switch and the breaker first, and tape the cable to the wall before you pull anything, so it cannot drop back inside.

The screen is dark, or it keeps restarting

With a C wire the thermostat has a proper return path. Without one it borrows a trickle through the heating or cooling wire, and that is what causes most Nest complaints.

Check 1Free, half an hour to two hours

Charge it by USB and see what happens

Pull the display straight off its base plate and look at the back. There is a USB port on it, mini or micro USB on older ones and USB C on the newest, but none on the cheaper Nest Thermostat. Plug it into a phone charger. Google says it usually takes about half an hour, and up to 2 hours if the battery is fully drained.

If it wakes up and works, the thermostat is fine and it simply ran out of power. That tells you the system is not keeping it charged, which is the next check rather than the end of it.

On the cheaper Nest Thermostat, change the two AAA cells for fresh alkaline ones first. Rechargeable cells sit at a lower voltage and cause exactly the same symptoms.

Then push the display back on until it clicks. They look seated when they are not, and a display that is not properly on its base has no power at all.

Check 2

Find out whether it has a C wire

Take the display off and look at the base plate. If there is a wire in the terminal marked C, it has a proper return path and the battery should stay charged.

If C is empty, the thermostat is borrowing power through the heating and cooling wires instead. That trickle is enough most of the time and not enough in cold weather when the system is busy, which is why these faults appear in December and not in June.

Three ways out, in order. Look for a spare conductor folded back in the wall at the furnace end, because cables are usually run with more wires than the job needed, and landing it on C at both ends costs nothing. Failing that, Google’s own power connector fits at the furnace and adds the path for about thirty dollars, or free when Google’s power test offers one. Failing that, a new cable.

The wiring side is the same on any thermostat: what each letter and colour does.

The screen works but the heat will not come on

Check 3

Four settings before you suspect the hardware

It is waiting on purpose. A “Delayed” or “Starts in” message means the thermostat is holding the system off for a few minutes, because Google says switching it on and off too often wears it. Wait it out before doing anything. If the message shows up often, Google lists that as a sign of a power problem.

It is in Eco. The word Eco on the screen means the thermostat thinks the house is empty and has dropped to its Eco temperature. Google says it switches back when it senses someone is home, or when you change the temperature in the app.

The schedule. A Learning Thermostat builds a schedule from what you did in the first week, and if that week was unusual, so is the schedule. It is worth reading rather than fighting.

The system type. In the installer settings it has to know whether it is driving a furnace or a heat pump, and on a heat pump whether the changeover is O or B. That gets wrong most often when somebody moves the thermostat or swaps a wire: what a heat pump needs set correctly.

Home and Away is switching it off

This deserves its own section because so many people take it for a broken thermostat.

The Nest watches the room with its own sensor and also listens to whether phones are at home. When it concludes the house is empty it goes to Eco, which can be several degrees below your normal setting. Come home to a cold house and it looks exactly like a fault.

It gets it wrong in predictable ways. A thermostat in a hallway nobody walks down. Somebody working from home in one room all day. A phone left on the wrong Wi-Fi or with location switched off.

You can switch the automatic side off entirely and keep the schedule, and on a house where somebody is home most days that is usually the right answer.

It says offline, but the heating still works

A Nest that has lost Wi-Fi is still a thermostat. The schedule and the settings live in the device, so the house keeps heating and only the app stops working.

Older models join 2.4 GHz networks only, so a router that merged both bands under one name can drop them. A new router, or a changed network name or password, means adding it again from scratch.

Worth knowing: a Nest with no C wire that is low on power will drop Wi-Fi first on purpose, to save what is left for running the heating. So an offline Nest and a battery problem are frequently the same story.

Resetting it, in order

Four resets, and they are not interchangeable. Start at the top, because the last one costs you the installer settings.

Work down them rather than jumping to the bottom. A restart fixes a frozen screen and loses nothing. Resetting the schedule clears a learned pattern that has gone wrong. Resetting Home and Away makes it forget when you are home or away, so it learns that again.

A factory reset wipes the system type, the wiring configuration and the account link. Before you do that, write down what the installer settings say, because putting the wrong system type back is a fault in itself.

None of the four does anything for a thermostat with no power. That is why they come last on this page.

Questions people ask

Why does my furnace click on for a second with nobody touching it?

That is the thermostat topping its battery up through the heating wire. It is the clearest sign there is that a C wire is missing.

The battery charges and then goes flat again within a week.

Same cause. Charging it by USB treats the symptom, and the C wire is the cure.

It worked for two years and only started this winter.

Cold weather makes the system run more, which leaves fewer gaps for a thermostat with no C wire to borrow power in. That is why these faults are seasonal.

Is it safe to leave it borrowing power?

Google says Nest thermostats are designed to do it, and in many homes it works. When it does not, it gives you a thermostat that restarts, an app that goes offline and a system that twitches. Most people fix it once and forget about it.

Nothing on the screen and the USB charge did nothing either.

Then look at the furnace rather than the thermostat. The panel not clicked shut, the switch on the cabinet and the fuse on the control board all leave a thermostat completely dead: the five free checks on a furnace.

My system is a boiler with only two wires.

Some two wire systems work and some do not, and a millivolt system that makes its own power from a pilot flame will not run one at all. Check the compatibility list against your wiring before buying.

Sources: Google Nest Help: "Learn about the common or C wire", "Nest thermostat USB port", "How to change Nest Thermostat batteries", "Restart or reset a Nest thermostat", "Learn about Eco Temperatures", "Learn to fix strange heating or cooling issues", "Learn about your thermostat's maintenance band" and "Nest thermostat technical specifications"