Nest thermostat: H73 or H153: OpenTherm not detected
The thermostat can’t detect your heating system’s OpenTherm connection.
Applies to: Nest Learning Thermostat and Nest Thermostat E with a Heat Link. What you see: a code.
Codes this page covers: H73 H153.
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 says to check that your heating system supports OpenTherm: its documentation, its manufacturer, or an OpenTherm logo on the system.
- Google says to check the Heat Link’s wiring against the diagrams in the installation guide, and to turn off the power to the Heat Link and your home at the fuse box before exposing or adjusting any wiring.
- Google says that if your system doesn’t support OpenTherm, choose On/Off boiler control instead in Settings, Equipment, Pro Setup.
- Google says that if the code comes after installation, check the power to your boiler hasn’t been turned off, and contact a local Nest Pro if it is switched on but not receiving power.
Google ties this to the boiler or heating system. What the symbols on a boiler’s own panel mean is on the boiler symbols page.
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.