Nest thermostat: 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.
Applies to: Nest Learning Thermostat, Nest Thermostat E and Nest Thermostat. What you see: a code.
Codes this page covers: E3 E4 E23 E24 E73 E74 E195.
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 1 below.
- Google says that if you get one of these codes in the summer you will likely need to contact an HVAC professional.
- Google says to check that your breakers are turned on, especially if you recently had construction or maintenance done on your home.
- Google’s own further checks are the thermostat wiring, the air filter, the drip pan or drain tubes and the HVAC fuse, each with the power off at the breaker first; it calls these advanced steps and says to get HVAC service from a professional if you find no problem.
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.