Thermostat displays E43 and app cannot connect to thermostat

Safety first: No electrical work on HVAC equipment is needed for this error.

Steps to troubleshoot:

• 1. Confirm internet access: Use a phone or computer on the same Wi‑Fi network to verify that you can browse external websites.
• 2. Reboot router and modem: Unplug both for 60 seconds, then plug them back in and wait for full connectivity.
• 3. Check router firewall: Log into your router and ensure that outbound connections are not blocked for the thermostat's IP or MAC address. Disable overly aggressive parental controls or security filters temporarily.
• 4. Verify time and date: Ensure your router or modem has the correct time and date; some secure connections fail if time is incorrect.
• 5. Reconnect thermostat: On the thermostat, go to Menu > Wi‑Fi, disconnect, then reconnect to the network. Confirm that the thermostat shows as connected.
• 6. Check Resideo cloud status: If other devices work but the app still cannot reach the thermostat, check Resideo/Honeywell Home service status online for outages.

When to call a professional: Persistent E43 with normal internet access may require assistance from Resideo support or your network administrator.