How do I fix Wi-Fi connection failed when adding a Honeywell T5 thermostat?
Answer
Put the T5 back into Wi-Fi setup, connect it to a 2.4 GHz network (not a 5 GHz-only or guest-isolated network), enter the password carefully in the Honeywell Home / Resideo app, and keep the phone on the same Wi-Fi during setup. Restart the router and thermostat if pairing still fails.
Follow these steps
Start Wi-Fi setup on the thermostat
On the T5, open Menu > Wi-Fi or Network and choose to connect/setup Wi-Fi so the thermostat is waiting for credentials (wording varies by firmware).
You should see: The thermostat is in setup mode for Wi-Fi.
Use a 2.4 GHz network
On your phone, join the 2.4 GHz SSID for your home Wi-Fi (or a combined SSID that allows 2.4 GHz). Avoid guest networks. Turn off VPN, then reopen the Honeywell Home / Resideo app and continue Add device / New Thermostat.
You should see: The app and thermostat are ready to share the same 2.4 GHz network.
Enter the password carefully
Type the Wi-Fi password in the app when prompted. Watch for caps lock and special characters. Wait for the thermostat to show connected rather than immediately restarting the flow.
You should see: The T5 shows a successful Wi-Fi connection instead of connection failed.
Power-cycle and retry if it still fails
Restart the thermostat (remove from the wall plate briefly if your model allows safe removal, or use the app/device reboot guidance), restart the router, then repeat setup. If it still fails, confirm MAC filtering is off and that the router is not blocking new devices.
You should see: Setup completes, or you identify a router setting blocking the thermostat.
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Extra tips
What to look for: “Connection failed” at Wi-Fi step; separate 2.4/5 GHz SSID names; app waiting on “New Thermostat”; thermostat Wi-Fi/Network menu
Before you change anything: Know your Wi-Fi password. Temporarily disable VPN on the phone. Use the official Honeywell Home / Resideo app. Place the phone near the thermostat during setup.
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