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

Note: The instructions in this article apply to heating and cooling systems common in the United States and Canada. If you live in the EU and need to troubleshoot your thermostat, please check Manually control heating when Heat Link is disconnected before you begin troubleshooting.

If your Google Nest thermostat isn't receiving power from your heating, ventilation, and air conditioning (HVAC) system, it will tell you there’s a problem. There are a few things you may notice on your thermostat:

• A “No power” or “No system power” alert should appear.
• A help code might also appear, such as M20, E448 or E298.
  • Some help codes can be dismissed, but they may appear again if the problem persists.
  • E help codes can’t be dismissed, so they won’t disappear until the issue is fixed.
• The Settings  icon will also have an exclamation point . If you hover over it, it will say ”Equipment error.”
• A “Low battery” or “Very low battery” alert may appear if the backup batteries continue to lose charge.
• Your thermostat may disconnect from Wi-Fi if it’s not receiving power, even if the battery level is good.
• Nest Learning Thermostat (4th gen) only: If you remove the display from the baseplate, the level at the top of the baseplate won’t illuminate if the thermostat isn’t receiving power.

A “No power” alert can be caused by the following:

• The thermostat wiring is incorrect.
• Your thermostat is connected to a system that requires a C or common wire, but that wire isn’t connected. Some systems, including heat-only, cool-only, zone-controlled, and heat pump systems, require a C wire or Nest Power Connector. Go to g.co/nest/cwire for details.
• The HVAC system is powered off at the circuit breaker, fuse box, or system switch, or there’s a power cut.
• The HVAC system is experiencing some other issue that prevents it from sending power to the thermostat wires.

Below are some steps to troubleshoot a “No power” alert on your thermostat:

This article only applies to the Nest Thermostat and Nest Learning Thermostat (4th gen).

How to tell which thermostat you have

Note: If your thermostat won’t turn on, or if your Nest Thermostat E or Nest Learning Thermostat (3rd gen or earlier) is having power issues, learn how to troubleshoot when your Nest thermostat won’t turn on.

Before you troubleshoot

• For Nest thermostat, change the batteries first so that you can control your system and stay comfortable while you troubleshoot. Nest Learning Thermostat (4th gen) has a rechargeable battery. If the thermostat turns off while you’re troubleshooting, charge the thermostat with a USB cable.
• If you recently turned power off at the circuit breaker of the heating and cooling system, make sure it’s back on.
• If you were installing the Nest Power Connector and there's an N260 help code, confirm that the cover is closed on your HVAC equipment. There is often a safety switch that prevents operation with the cover removed.

Troubleshoot

1. Refer to the equipment information

To narrow down possible causes of a “No power” alert:
On your thermostat, navigate to the Settings .

1. Nest Thermostat: Select Equipment .

• A “No power” or “No system power” alert should appear.
• An Error code (such as E## or N##) might also appear.

2. Nest Thermostat (4th gen): Equipment   Wiring Info.

• A “No power” alert should appear.
• An Error code “E448: No system power” might also appear.

2. Check the wiring

On your thermostat, go to Settings   Equipment  Wiring info. Check for wires that appear grey. This means that the wire isn't sending voltage to your thermostat. Some wires, like a C or R wire, need to send voltage to keep your thermostat powered on.

If all the wires are grey, the system may be powered off or experiencing some other power-related issue. Check that the system is powered on at the circuit breaker, fuse box, or system switch.

If any wire is grey, it isn’t electrically detected by your thermostat. This usually happens because the wire isn’t properly connected. It might also mean that the wire was incorrectly entered in the app. Check the system wires to make sure they are correctly entered and properly connected.

Important: You must turn the system off before checking the system wires. This will help prevent damage to them and your thermostat.

1. Turn power off for the system. The power switch will usually be in the circuit breaker, fuse box, or system switch. In some homes, resemble a light switch near the heating or cooling equipment.

Note: There may be one switch for heating and one for cooling, or a single switch for both.

2. Pull your thermostat away from its base to remove it.

• Check that the wires are correctly identified. If they’re not, you’ll need to set up your thermostat again with the correct wiring info.

Important: For the Nest Learning thermostat (4th gen), the LED should be turned off before you proceed to step 3.

3. To disconnect each wire, push down its connector tab and gently pull out the wire. Then, do the following as needed:

• Straighten the wire end.
• Strip the wire to expose ⅓" to ½" of copper.
• Seat the wire fully into the right connector.

4. After all the wires are connected, re-attach your thermostat to its base.

5. Turn the power for the system back on.

6. After a few moments, use your thermostat to go to the Menu view Equipment , then check if the issue is resolved.

3. Connect a C wire or the Nest Power Connector

Your thermostat may require a C wire or the Nest Power Connector to provide enough consistent power.

1. Check whether a C wire or spare wire is available. If you find one, connect it to your thermostat’s C connector.
2. If a C wire isn’t available, install the Nest Power Connector or pull a new C wire.

Note: A new C wire installation generally requires a pro.

Learn about the common or C wire

4. Contact a local pro

If you think wires are incorrectly connected to your thermostat and want additional help, contact Nest support. You can also read more steps on how to troubleshoot Nest thermostat help codes.

If you think there may be a problem with the heating and cooling system, or your system requires a C wire, contact a local pro.