A Nest thermostat can be powered by your HVAC system through its wires and backed up by a battery, and Google's No power article explains what goes wrong: a No power or No system power alert, sometimes with help codes M20, E448 or E298, an Equipment error mark on the settings icon and a low or very low battery alert if the backup battery keeps draining.
The checks in more detail
The causes are wrong wiring, a system that needs a C or common wire that is not connected (some heat-only, cool-only, zone-controlled and heat pump systems), the HVAC powered off at a breaker, fuse or system switch, a power cut or another fault preventing power to the wires. Before you troubleshoot, change the batteries on a Nest Thermostat, or charge a Nest Learning Thermostat (4th gen) with a USB cable since its battery is rechargeable. Then follow Google's steps: check the equipment information, check the wiring, connect a C wire or the Nest Power Connector and contact a local pro.
Google's hot weather article covers power issues with codes E3, E4, E23, E24, E73, E74 or E195, and its low battery article covers changing batteries. If a Power Connector install shows an N260 code, close the equipment cover, since a safety switch stops operation. A Power Connector costs about $24.99. Take a clear photo of the wires on the thermostat base before you change anything so you can always go back to the original layout. A thermostat that works in mild weather and fails in summer or winter points at a power supply that cannot meet peak demand.
If a pro installs a C wire, ask them to label it and photograph the final wiring. A wiring photo, the old thermostat's wire labels and the model of the HVAC equipment together let a technician identify a missing C wire in minutes rather than an hour. The Nest Learning Thermostat 4th gen has a rechargeable battery that charges through a USB cable, and Google suggests using that to keep it running while you troubleshoot power, since a flat battery is the reason a thermostat shuts down mid-fix.
Differential
Likely causes of “Nest thermostats has power or battery problems”, and how to tell them apart
- 1
A missing C wire
What separates it The thermostat shows no power and the wifi drops because there is no C wire supplying it
How to confirm Connect a C wire, or fit a Power Connector, so the thermostat receives steady power
- 2
Wrong wiring
What separates it The thermostat shows a No power alert because the wiring is not connected correctly
How to confirm Check the wiring against your photo and the labels, and reseat any wire that is loose
- 3
HVAC off at a breaker
What separates it The thermostat shows No system power because the heating system is switched off at a breaker
How to confirm Restore power to the heating and cooling system at the breaker and the equipment switch
- 4
A draining battery
What separates it The thermostat shows low battery alerts because its battery is draining
How to confirm Change or charge the battery, and the alerts should stop once it has power again
- 5
A cover safety switch
What separates it The thermostat shows an N260 code because a safety switch under the equipment cover is open
How to confirm Close the equipment cover properly, so the safety switch closes, then check the code clears
Before you touch a setting
Record this first — changing settings destroys the evidence
- The alert and the help code that the thermostat shows
- A photo of the wiring behind the thermostat
- Whether a C wire exists in the wall
Try in this order
0 of 7 completedRead the alert and code
ReversibleRead the alert and the code, such as M20, E448 or E298, and look it up in Google's article.
Change or charge the battery
ReversibleChange or charge the battery, which keeps the thermostat running while you find the cause.
Check equipment power
ReversibleCheck the equipment power at the breaker and the switch, since the system may be off.
Check the wiring
ReversibleCheck the wiring, taking a photo first so that you can put every wire back correctly.
Add a C wire or Power Connector
ReversibleAdd a C wire or a Power Connector if the system needs one to give steady power.
Close the equipment cover
ReversibleClose the equipment cover if the code is N260, so that the safety switch is closed.
Contact a pro
Reversible
Model-specific
What is true of this model and not of its siblings
- 4th gen
- Rechargeable battery, USB charging.
- Nest Thermostat
- Backup batteries you replace.
Stop boundary
When this stops being a self-service repair
- !If the No power alert persists after wiring checks, contact a local pro
- !If the code is E-type, it cannot be dismissed until fixed
- !If unsure about wires, contact an HVAC pro
Evidence ledger
Sources behind this answer
No power alerts, C wire and Power Connector.
Low battery.
Hot weather power.
Blank screen.
The company and its products.
Exact questions
Common questions
What causes No power?+
No power is caused by wiring, a missing C wire, or the heating system being switched off.
Are E codes dismissable?+
No, E codes cannot be dismissed, and they stay until you fix the cause of the problem.
Is the 4th gen battery rechargeable?+
Yes, the battery on the 4th generation model is rechargeable through its USB port.
What is a Power Connector?+
A Power Connector is a substitute for a C wire, and it provides steady power to the thermostat.
How much is a Power Connector?+
A Power Connector costs $24.99, according to Google's store, though prices can change over time.
Can hot weather cause power codes?+
Yes, hot weather can cause power codes, since heat can make a system's safety switches trip.
Should I call a pro?+
Yes, call a professional if you are unsure, since wiring mistakes can damage the system.
Should I test after a change?+
Yes, test after any change by setting a target temperature and checking that the heating or cooling responds.