Honeywell Home thermostats show numbered alerts rather than lettered error codes — and most of them are reminders, not faults. 181 and 182 mean change a filter, 405 and 407 mean change the batteries, and 171 just wants the clock set. The ones that matter are 164 (the heating or cooling equipment is reporting a fault), 170 and 173 (the thermostat itself has failed), and the Wi-Fi group in the 300s and 400s.
Where the alert appears
On T-series and Lyric thermostats an alert shows as a number on the home screen, usually alongside a warning triangle, with a short description and a Dismiss option. Older round and rectangular Honeywell mercury-free thermostats have no alert numbers at all — they signal problems through the display itself (a blank screen, a flashing Cool On or Heat On indicator during the compressor protection delay, or a low battery icon).
Alert numbers are model-specific. The list below covers the widely used T6 Pro family; if your thermostat shows a number that is not here, check the installation instructions for your exact model number, which is printed on the back of the thermostat behind the wall plate.
Honeywell alert codes and what they mean
| Alert | Official meaning | What to do |
|---|---|---|
| 164 | The L terminal is energised — the heat pump is reporting a fault to the thermostat | Call a heating and cooling professional |
| 170 | Thermostat memory failure | Replace the thermostat |
| 171 | Reminder to set the date and time | Set the clock so schedules run correctly |
| 173 | Internal temperature sensor failure | Replace the thermostat |
| 177 | Invalid reading on a wired indoor sensor | Check the remote sensor and its wiring and settings |
| 178 | Invalid reading on a wired outdoor sensor | Check the outdoor sensor and its wiring and settings |
| 181 / 182 | Replace filter reminder | Fit a new furnace filter, then dismiss |
| 405 | Low battery | Fit fresh AA batteries |
| 407 | Critically low battery | Fit fresh AA batteries now — the display shuts down otherwise |
Wi-Fi and connection alerts
On Wi-Fi models a second group of numbers reports connectivity rather than hardware. Reported meanings across Honeywell’s Wi-Fi thermostat range are:
- 168 — Wi-Fi radio error inside the thermostat.
- 388 — the thermostat is not registered to an account; finish setup in the app to get remote access and outdoor temperature.
- 399 — connected to the router but with no route to the internet.
- 400 — Wi-Fi signal lost; the thermostat will retry on its own.
None of these stop the thermostat controlling your heating and cooling — it keeps running its schedule locally. They only affect the app, remote access and outdoor temperature display. If yours will not join the network at all, our guide on a thermostat not working and the setup checks in that article are the place to start.
How to clear an alert
Do not pull the thermostat off the wall to “reset” it while the system is powered. The terminals behind it carry 24 volts, and shorting the R and W or R and Y terminals can blow the control board fuse in your furnace or air handler — turning a dismissible reminder into a real repair. Switch the system off at its service switch or breaker first.
- Read the number before you dismiss it
Dismissing an alert removes the only record of what happened. Photograph the screen. Reminder alerts (171, 181, 182) are meant to be dismissed after you have done the task; fault alerts are not.
- Change the batteries for 405 and 407
Use fresh alkaline AA batteries, not rechargeables — the lower cell voltage of rechargeables can leave the thermostat in a permanent low-battery state. Replace both at once. If the thermostat is powered by a C-wire and still reports low batteries, the C-wire connection may not actually be live.
- Replace the filter, then dismiss, for 181 and 182
These are timer-based reminders, not measurements — the thermostat has no way to see your filter. It restarts the count when you dismiss the alert, so dismiss it only after the filter is actually changed. Our guide on how often to change an HVAC filter covers realistic intervals.
- Set the clock for 171
A thermostat with no date and time cannot run a schedule, so it holds a single setpoint instead. This alert commonly appears after a long power cut or a battery change on a non-Wi-Fi model.
- Check sensor wiring for 177 and 178
These alerts refer to an optional wired remote indoor or outdoor sensor. Confirm the sensor is still connected and undamaged, and that the installer setting matches the sensor type fitted — a 10K sensor configured as 20K reads as invalid.
- Treat 164 as an equipment fault, not a thermostat fault
Alert 164 means the heat pump has energised its L terminal to signal a problem of its own. The thermostat is the messenger. The fault is in the outdoor unit, and diagnosing it needs a technician with gauges.
If an alert points at the equipment rather than the thermostat, resist the temptation to open the appliance cabinet. Gas furnaces combine a gas valve, a hot surface igniter and mains voltage in one enclosure — that is a licensed technician’s work. Air handlers and heat pumps carry capacitors that hold a lethal charge with the power off. Your safe boundary is the thermostat, its batteries, the filter and the breaker.
When to call a professional
Call a heating and cooling professional for alert 164, for any alert that returns immediately after you have done what it asks, and for a thermostat that keeps losing power or resetting — the last of those often means the C-wire or the 24V transformer, not the thermostat. Alerts 170 and 173 mean the thermostat is finished; it can be swapped by a competent DIYer who photographs the wiring first, but a heat pump or multi-stage system is worth handing to a professional so the installer settings are configured correctly.
