Honeywell thermostat: Alert code 164: heat pump problem
The L terminal is energized at the equipment: the heat pump is telling the thermostat there is a problem with the heat pump.
Applies to: T6 Pro thermostats. What you see: a code and a symbol.
What Honeywell Home says to do
- Honeywell Home says some alerts cannot be snoozed or cleared, and if so to call your local heating and cooling professional, as the system may need service.
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.