Honeywell thermostat: Alert code 173: internal sensor failure
An alert to replace the thermostat because its internal sensor has failed.
Applies to: T6 Pro thermostats. What you see: a code and a symbol.
What Honeywell Home says to do
- Honeywell Home says to replace the thermostat.
Source
Honeywell Home’s own help page, read on the date shown.
- Honeywell Home (Resideo) documentation: 7. Alerts and Reminders — T6 Pro Installation Manual — T6 Pro thermostats (codes 177 and 178 not on the TH6210U). docs.honeywellhome.com Checked 2026-09-29.
A similar symbol on other thermostats
The same symbol on another maker’s thermostat can mean something else.