Nest thermostat: 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.
Applies to: All Nest thermostats; the same fault has a different E number on each generation. What you see: a code.
What Google says to do
Wiring and power. Google ties this, on at least some systems, to the wiring, the power supply or the C wire. The steps here are only Google’s own, as its page states them; semiatic gives no wiring instructions of its own. Google’s own line on calling a professional is step 3 below.
- Google warns you to turn off the power to your system at the fuse box before exposing or adjusting any wiring.
- Google says to select your exact code in its help-code table and follow the steps it links for that code.
- Google says you can also contact a pro for other issues, including HVAC.
Source
Google’s own help page, read on the date shown.
- Google Nest Help: Troubleshoot Nest thermostat help codes — Nest Learning Thermostat (1st to 4th gen), Nest Thermostat E and Nest Thermostat; H codes only where a Heat Link is installed. support.google.com Checked 2026-09-29.