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Thermostats · Nest Thermostats

Nest Thermostat Not Working? 7 Causes and How to Fix Them

Nest thermostat not working or not turning on? Read the light, charge the battery, reseat the display, check the wires and C wire, then restart it.

By Evrenus Fırat PolatEditor & ResearcherPublished Checked against 1 manual + 6 manufacturer pages + 1 government source

Quick answer

A Nest thermostat that stops working is usually low on power. A yellow or blinking red light means the battery is charging, which can take up to an hour. With no light, reseat the display, check the breaker and fuse box, and check that each wire is fully inserted. Some systems need a C wire or Nest Power Connector.

Check these first:

  1. Yellow or blinking red light?
  2. Display clicked onto its base
  3. Battery model: fresh AAA batteries
Time to fix
5–60 min
Difficulty
Easy, 1 of 4
Safety
WarningElectrical
On this page · 13 sections

A Nest thermostat runs on a small built-in battery that the heating and cooling system keeps charged, except the battery-powered Nest Thermostat, which uses two AAA cells.⁠[1] When it stops working, Google's own troubleshooting starts with power: the light on the thermostat, the display's seat on its base, the breaker and the wires.⁠[1] This guide follows that order and tells you which steps differ by model. For how the Nest models differ, see Nest thermostat basics.

Before you start

Find your model. Google splits its steps three ways: the Nest Learning Thermostat (4th gen); the Nest Thermostat E and Learning Thermostat 3rd gen or earlier; and the battery-powered Nest Thermostat.⁠[1]

Shut-offs

  • Power

    The breaker or fuse for the furnace, air handler or boiler, or the system switch next to the equipment.

Before any reset, write down your wiring from Settings, then Equipment. Google says to note it, and settings such as heating type and safety temperature, before a factory reset.⁠[3]

You'll need

  • LED work flashlight
  • AAA alkaline batteries
  • Furnace air filter

What the light on your Nest means

Google uses the light to tell you what the thermostat is doing.⁠[1]

LightModelsMeaning and first step
Dark yellowLearning Thermostat (4th gen)Battery very low but charging; wait up to an hour⁠[1]
Blinking redThermostat E, Learning 3rd gen or earlierBattery charging; wait up to an hour⁠[1]
Blinking greenThermostat E, Learning 3rd gen or earlierUpdating or restarting; wait a minute or two⁠[1]
Yellow or red that never stopsWired modelsLow power; check the power readings and C wire⁠[1]
No lightAll modelsReseat the display, then check system power⁠[1]
Fig. 1How thermostat wires connect to the furnace and AC
How thermostat wires connect to the furnace and ACWiring schematic of a wall thermostat connected to a gas furnace control board and an outdoor AC unit. Twelve numbered parts, following the power: 1 service switch on the 120-volt supply below the furnace; 2 door switch on the blower door; 3 transformer on the control board, making 24 volts; 4 fuse on the board; then the thermostat terminals, top to bottom: 5 R wire bringing 24-volt power; 6 Rc jumper joining R and Rc; 7 C wire, the common return; 8 W wire for heat; 9 Y wire for cooling; 10 G wire for the fan; 11 O/B wire, used only by heat pumps and empty here; 12 outdoor unit, fed by the Y and C wires from the furnace terminal strip. Dashed lines are 24-volt thermostat wires; thick solid lines are 120-volt power.ThermostatRRcCWYGO/BFurnaceRCWYGYC123456789101112Service switchDoor switchTransformerFuseR wireRc jumperC wireW wireY wireG wireO/B wireOutdoor unit

  1. Service switch and breaker DIY

    The furnace runs on household power from a breaker, usually through a switch that looks like a light switch near the unit. If either is off, the thermostat loses its 24-volt supply too.[6][7]

  2. Blower door switch DIY

    A switch pressed by the blower access panel. With the panel off or loose, it cuts power to the control board, so nothing runs and a wired thermostat may go blank. Never tape it closed.[7][6]

  3. Transformer Pro only

    Turns the furnace’s 120-volt supply into the low voltage the thermostat circuit uses. Honeywell says thermostats work on 20 to 30 volts AC; a reading outside that range is a job for a professional.[7][6]

  4. Control board fuse Pro only

    Many boards protect the 24-volt circuit with a small fuse (3 amps on the Bosch furnace). A short between thermostat wires during an install can blow it, and then the thermostat and the system both go dead.[7]

  5. R (power) DIY with care

    Brings 24-volt power from the transformer up to the thermostat. Every call for heat, cooling or fan starts by connecting R to another wire.[6][8]

  6. Rc and the R/Rc jumper DIY with care

    With one R wire, R and Rc must be joined by a metal jumper or a selector set to 1 Wire. With separate R and Rc wires, the jumper comes out or the selector goes to 2 Wire.[6][8]

  7. C (common) DIY with care

    The return path to the transformer. Smart thermostats need it for steady power; if your old thermostat had no C wire, it is probably not connected at the furnace either. Running a new one generally needs a pro.[6][2]

  8. W (heat) DIY with care

    When the thermostat connects R to W, the furnace starts its heating sequence. On a heat pump, heat is called through Y and O/B instead.[8][7]

  9. Y (cooling) DIY with care

    Calls for cooling. At the furnace, Y connects on to the outdoor unit, where it starts the compressor.[7]

  10. G (fan) DIY with care

    Runs the furnace blower by itself, as when the fan switch is set to On. In heating and cooling, the board also starts the blower on its own timing.[7]

  11. O/B (heat pumps only) DIY with care

    Heat pumps use O/B together with Y to switch between heating and cooling. On a furnace with a plain AC like this one, the terminal stays empty.[8][2]

  12. Outdoor AC unit Pro only

    Its low-voltage pair (Y and C) lands on the furnace terminal strip, not on the thermostat. A cooling call on Y energizes the compressor.[7]

  • Before you touch a wire. Turn the system off at its breaker or switch first, and note which terminal each wire came from. Wire colors are a habit, not a rule; go by the terminal letter.[2][6]
A wired Nest charges from R and C. Heat-only, cool-only, zoned and heat pump systems may need a C wire or a Nest Power Connector.

  1. Battery charging after a power dip

    Most commonDIYTime: 10–60 min

    A dark yellow or blinking red light while the battery recharges.
    Fix it: Battery charging after a power dip

  2. Display off its base or system power off

    CommonDIYTime: 5 min

    No light because the display is not seated or the breaker is off.
    Fix it: Display off its base or system power off

  3. Flat AAA batteries on the Nest Thermostat

    CommonDIYTime: 5 min

    The battery-powered model goes dark.
    Fix it: Flat AAA batteries on the Nest Thermostat

  4. Loose or misplaced wire

    CommonDIY with careTime: 20 min

    E or N help codes, or grey wires in the wiring screen.
    Fix it: Loose or misplaced wire

  5. System needs a C wire or Nest Power Connector

    Less commonDIY with careTime: 30 min

    Heat-only, cool-only, zoned and heat pump systems may not charge the battery.
    Fix it: System needs a C wire or Nest Power Connector

  6. Frozen during an update

    Less commonDIYTime: 5 min

    Blinking green for more than 15 minutes, or a stuck screen.
    Fix it: Frozen during an update

  7. Fault in the heating and cooling system

    Less commonPro only

    A blown control-board fuse or another fault stops power to the thermostat.
    What a pro does: Fault in the heating and cooling system

Let the battery charge

On a Learning Thermostat (4th gen), a dark yellow light means the battery is very low but charging. Google says that can take several minutes or up to an hour, and if it is still off after an hour, another problem likely exists.⁠[1] On the Thermostat E and older Learning models, a blinking red light means the same thing.⁠[1]

  1. Leave the thermostat on its base for up to an hour

    • Time: 60 min

    Google says charging can take several minutes, or up to an hour if the battery is very low.⁠[1]

    You'll know it worked when: The light stops and the screen turns on.

  2. Charge the display by USB to speed things up

    • Time: 15 min

    Pull the display off and charge it through the port on its back from a wall charger, not a computer or another unpowered USB port. The 4th gen uses USB-C; the Thermostat E and older Learning models use mini or micro USB. Then press the display back on until it clicks.⁠[1]

    You'll know it worked when: The display has some charge and clicks back onto its base.

  3. Read the power numbers if the light keeps coming back

    • Time: 5 min

    Open Settings, then Technical Info, then Power, and compare the values with Google's ranges.⁠[1]

    You'll know it worked when: You have the Voc, Vin and lin values written down.

USB charging is a stopgap. Google says it does not fix the underlying problem, so if the battery keeps running down, work through the wiring and C wire sections.⁠[1]

Still not fixed? Next likely cause: Display off its base or system power off

Reseat the display and check system power

With no light at all, Google's first two checks are the display and the power to your system.⁠[1]

  1. Press the display straight onto its base until it clicks

    • Time: 1 min

    Clear any cable that keeps it from seating.⁠[1]

    You'll know it worked when: The display sits flat and the screen wakes.

  2. Turn the breaker or system switch fully on

    • Time: 3 min
    • LED work flashlight

    Google says to switch the breaker or switch fully to On.⁠[1]

    You'll know it worked when: The breaker and any system switch are firmly on.

  3. Check the fuse box for a blown fuse

    • Time: 3 min

    If your system runs from a fuse box, Google says to check it for a blown fuse.⁠[1]

    You'll know it worked when: Every fuse for the system is intact.

  4. Close the equipment cover

    • Time: 2 min

    Google notes there is often a safety switch that prevents operation with the equipment cover removed.⁠[2]

    You'll know it worked when: The furnace or air handler cover is fully closed.

Power cuts and recent maintenance are common triggers. Google suggests checking breakers first after construction or service work.⁠[4]

Still not fixed? Next likely cause: Flat AAA batteries on the Nest Thermostat

Replace the AAA batteries on the Nest Thermostat

The battery-powered Nest Thermostat has no rechargeable battery. Google's steps are to reseat both batteries first, then replace them.⁠[1]

  1. Take the thermostat off its base and reseat both batteries

    • Time: 2 min

    A battery that has shifted can stop the thermostat.⁠[1]

    You'll know it worked when: Both batteries match the + and - marks.

  2. Fit two new 1.5 V AAA alkaline batteries

    • Time: 2 min
    • AAA alkaline batteries

    Google says to use batteries from a known manufacturer.⁠[1]

    You'll know it worked when: The thermostat turns on after you press it onto the base.

  3. Press it onto the base until it clicks

    • Time: 1 min

    Google says it takes a few moments to turn on and restart.⁠[1]

    You'll know it worked when: The thermostat restarts within a few moments.

Fresh batteries are not enough on some systems. Google says heat-only, cool-only, zone-controlled and heat pump systems may need a C wire or Nest Power Connector, even for this model.⁠[1]

Still not fixed? Next likely cause: Loose or misplaced wire

Check the wires and help codes

Google says E help codes are critical: the thermostat won't let you control heating or cooling until the cause is fixed.⁠[4] E73 means no power on the Rc (cooling power) wire and E74 means no power on the Rh (heating power) wire.⁠[4] On the Nest Thermostat and 4th gen, a grey wire in Wiring Info is not sending voltage to the thermostat.⁠[2]

  1. Open the wiring screen

    • Time: 2 min

    Go to Settings, then Equipment (on a 4th gen, then Wiring Info).⁠[2] All grey means the system is off or has a power fault; some grey means loose or misentered wires.⁠[2]

    You'll know it worked when: You see which wires are grey, if any.

  2. Turn the system power off

    • Time: 2 min

    Do this before you take the display off.⁠[2]

    You'll know it worked when: The breaker, fuse or system switch is off.

  3. Reseat each wire until its button stays down

    • Time: 10 min

    Google says each exposed end should be clean and straight, about 3/8 inch (1 cm) long, and fully inserted.⁠[1]

    You'll know it worked when: Every connector button stays fully pressed down.

  4. Match the wires to the app's wiring diagram

    • Time: 5 min

    Wires in the wrong connectors can leave the thermostat with no light at all.⁠[1]

    You'll know it worked when: Each wire is in the connector the diagram shows.

  5. Restore power and check the code

    • Time: 5 min

    Push the display onto the base until it clicks, then turn the power back on.⁠[1]

    You'll know it worked when: The code clears, or you have its number for support.

Google adds that an E3, E4, E23, E24, E73, E74 or E195 code in summer usually needs an HVAC professional.⁠[4]

Still not fixed? Next likely cause: System needs a C wire or Nest Power Connector

Add a C wire or Nest Power Connector

Most wired Nests charge their battery from the system's control wires. Some systems can't do that well. Google lists heat-only, cool-only, zone-controlled and heat pump systems as ones that may need a C wire or a Nest Power Connector.⁠[2]

  1. Look for a C wire or a spare wire

    • Time: 5 min

    With the power off, check behind the base. A C or spare wire goes on the C terminal.⁠[2]

    You'll know it worked when: You know if a C or unused wire is in the wall.

  2. Plan a Nest Power Connector or a new C wire

    • Time: 5 min

    With no spare wire, Google says to install a Nest Power Connector or run a new C wire, which generally needs a pro.⁠[2]

    You'll know it worked when: You know which fix fits your system.

A spare wire only works once it is also connected at the furnace or air handler end. That end is inside the equipment, so book a pro if you have not worked on a control board before.

Still not fixed? Next likely cause: Frozen during an update

Restart a frozen Nest

A blinking green light on a Thermostat E or older Learning model means it is updating, starting or restarting, which usually takes a minute or two. Unresponsive for more than 15 minutes means it may have frozen.⁠[1]

  1. Reseat the display first

    • Time: 1 min

    Google suggests this before a restart.⁠[1]

    You'll know it worked when: The display clicks back on and responds.

  2. Restart from the menu if the screen works

    • Time: 2 min

    4th gen: Settings, then Restart. Thermostat E and 3rd gen or earlier: Settings, then Reset, then Restart.⁠[3]

    You'll know it worked when: The thermostat restarts and keeps its settings.

  3. Force a restart on a frozen ring model

    • Time: 2 min

    Press the thermostat down until the screen turns off, about 10 seconds, then press and release the ring.⁠[3]

    You'll know it worked when: The screen comes back on and finishes the restart.

On the battery-powered Nest Thermostat, a manual restart means taking out both AAA batteries, putting them back and pressing it onto its base.⁠[3] A restart keeps your settings. Leave a factory reset for last; afterwards you have to set the thermostat up and add it to your home in the app again.⁠[3]

Still not fixed? Next likely cause: Fault in the heating and cooling system

When the heating and cooling system is the problem

Google's no-power article lists an HVAC fault that stops the system from sending power to the thermostat as one cause of a dead Nest.⁠[2] A blown fuse on the control board can block power too, and Google says to contact a professional for it.⁠[1]

  1. Check the air filter

    • Time: 5 min
    • Furnace air filter

    Google says a clogged filter can make the system overheat and shut off.⁠[1] ENERGY STAR says to change a dirty filter, or at least every 3 months.⁠[5]

    You'll know it worked when: The filter is clean or a new one is in.

  2. Test with your old thermostat if you kept it

    • Time: 20 min

    Google says that if the system works with the old thermostat, the Nest may be damaged.⁠[1]

    You'll know it worked when: You know if the system runs with the old thermostat.

  3. Call an HVAC pro for a board or fuse fault

    • Time: 5 min

    Leave the control board and its fuse to them.⁠[1]

    You'll know it worked when: A technician is booked with the light pattern and help code in hand.

For heating that runs but blows cool air, see heater blowing cold air. For Wi-Fi drops, see Nest thermostat offline or not connecting.

Still not fixed? See when to call a pro

Parts and tools for this fix

You rarely need parts for a Nest that will not work. The two that come up are fresh AAA cells and a clean filter.⁠[1]

When to call a pro

Google's last step on every Nest power article is a local professional, because the cause may be in your heating and cooling system.⁠[1]

Other brands share many of these steps; compare the ecobee thermostat that will not turn on, or read thermostat wiring and basics.

Frequently asked questions

FAQ

How long does a dead Nest take to charge?
Google says several minutes or up to an hour. If it is still off after an hour, another problem is likely.⁠[1]
What does E74 mean on a Nest thermostat?
No power was detected on the Rh wire, the heating power wire. E73 means no power on the Rc wire.⁠[4]
Will restarting my Nest erase my settings?
No. Google says a restart keeps all your settings; only a factory reset restores the defaults.⁠[3]
Can I charge my Nest with a laptop?
Google says to use a wall charger, not a computer or other unpowered USB port, for the 4th gen.⁠[1]

Sources

  1. Google Nest Help · Manufacturer support

    “Troubleshoot when your thermostat won't turn on” · accessed

  2. Google Nest Help · Manufacturer support

    “Troubleshoot a "No power" alert on a Nest thermostat (E298, E448 or M20 help code)” · accessed

  3. Google Nest Help · Manufacturer support

    “Restart or reset a Nest thermostat” · accessed

  4. Google Nest Help · Manufacturer support

    “Troubleshoot Nest thermostat help codes” · accessed

  5. ENERGY STAR (U.S. EPA) · Government

    “A Guide to Energy-Efficient Heating and Cooling” · PDF · p. 6 (air filter) · accessed

  6. Honeywell Home (Resideo) · Manufacturer support

    “Why won't my thermostat screen power up?” · accessed

  7. Bosch Thermotechnology Corp. · Manufacturer manual

    “Bosch 96% AFUE Gas Furnace BGH96: Installation, Operation & Maintenance Manual” · PDF · §10 p. 39 (door switch), §10.5 p. 41 (24 V wiring, fuse), Fig. 35-36 pp. 42-43, §11.4 p. 44 · accessed

  8. Honeywell Home (Resideo) · Manufacturer support

    “Why is my Heating not Working on my RTH6500WF Smart Series Programmable Thermostat?” · accessed

Google Nest is a trademark of its owner. Before You Call a Pro is not affiliated with or endorsed by Google Nest.

How this guide was made

This guide was drafted with AI assistance from the manufacturer and safety sources listed above, then checked against those sources in an AI-assisted review. No licensed technician reviewed it. Last reviewed . Our methodology

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