Google Nest thermostat: what its codes and messages mean

A Nest thermostat puts a help code on its screen when it finds a problem: a letter for the kind of problem and a number. Google’s own table says E codes are critical and stop you controlling heating or cooling, N codes are warnings you can often dismiss, W codes are Wi-Fi, and H codes appear only on UK and European installations that use a Heat Link by the boiler.

The code appears on the thermostat’s round screen, usually with a short message and a nest.com address. Google lists the same code number under different model generations, so the letter matters more than the exact number.

1. E73 (also E3, E4, E23, E24, E74, E195): power error

   Your heating and cooling system has stopped sending power to the thermostat. Google says this often happens when the system breaks down in hot summer weather; if it goes on, the thermostat’s battery drains and it may disconnect from the internet or shut down.

   Nest Learning Thermostat, Nest Thermostat E and Nest Thermostat. 3 steps from Google on its page. About wiring or power: only Google’s own instruction is repeated.

2. Any E code (E1 to E452): critical fault

   E codes are critical and must be fixed as soon as possible: the thermostat will not let you control heating or cooling, and the message will not go away until the cause has been fixed.

   All Nest thermostats; the same fault has a different E number on each generation. 3 steps from Google on its page. About wiring or power: only Google’s own instruction is repeated.

3. N72 and other N codes: low power to the thermostat

   One of the wires in your thermostat is receiving power, but less power than expected.

   Nest Learning Thermostat, Nest Thermostat E and Nest Thermostat. 3 steps from Google on its page. About wiring or power: only Google’s own instruction is repeated.

4. M6 or N412: your system needs maintenance

   There is a problem with your heating and cooling system itself, not the thermostat.

   Nest Learning Thermostat, Nest Thermostat E and Nest Thermostat. 1 step from Google on its page.

5. W1: can’t stay connected to Wi-Fi

   The thermostat can’t stay connected to your Wi-Fi network. Google says your router may not fully support a Wi-Fi standard known as power saving mode.

   All Nest thermostats. 1 step from Google on its page.

6. W2: Failed to connect (router security)

   Failed to connect: your Wi-Fi router may have security settings turned on that stop the thermostat connecting.

   All Nest thermostats. 1 step from Google on its page.

7. W3 or W4: Failed to connect (no IP address or no internet)

   Failed to connect. With W3 your router is not giving the thermostat a network IP address; with W4 the thermostat is connected to your wireless network but not to the internet.

   All Nest thermostats. 1 step from Google on its page.

8. W5: can’t connect to Wi-Fi (possible hardware fault)

   The thermostat can’t connect to Wi-Fi. Google says this can indicate a hardware malfunction, and the thermostat may need to be replaced.

   Nest thermostats with a ring and Quick View menu. 3 steps from Google on its page.

9. H1, H71 or H151: can’t reach the Heat Link

   The thermostat can’t connect to your Heat Link to control your heating. The Heat Link’s status light is yellow while it is disconnected from the thermostat.

   Nest Learning Thermostat and Nest Thermostat E with a Heat Link. 4 steps from Google on its page.

10. H2, H72 or H152: the thermostat lost power

    Your thermostat has lost power.

    Nest Learning Thermostat and Nest Thermostat E with a Heat Link. 4 steps from Google on its page. About wiring or power: only Google’s own instruction is repeated.

11. H73 or H153: OpenTherm not detected

    The thermostat can’t detect your heating system’s OpenTherm connection.

    Nest Learning Thermostat and Nest Thermostat E with a Heat Link. 4 steps from Google on its page. About wiring or power: only Google’s own instruction is repeated.

12. H155: Heat Link went offline, batteries low

    Your Heat Link went offline when its batteries were low.

    Nest Thermostat E with a battery-powered Heat Link. 2 steps from Google on its page.

13. H156: OpenTherm available but not selected

    Your system is wired for and compatible with OpenTherm, but the OpenTherm option wasn’t selected during setup.

    Nest Thermostat E with a Heat Link. 2 steps from Google on its page.

14. H157 or H158: Heat Link removed from the app

    H157: the Heat Link has been removed from the app. H158: it has been removed from your account and the thermostat has disconnected from the app.

    Nest Thermostat E with a Heat Link. 3 steps from Google on its page.

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