the app says my thermostat is offline but it's working fine on the wall. how do i resync it?

Diagnosis

The "Offline" status in the Resideo/Honeywell Home app, while the wall unit continues to regulate temperature, indicates a break in the handshake between the T6 Pro hardware and the cloud server. This is rarely a failure of the thermostat's internal logic; rather, it is a network layer failure. The thermostat is functioning as a "dumb" device, executing its programmed schedule locally, but it has lost its IP lease or the socket connection to the remote server has timed out. This typically occurs due to a DHCP conflict where the router assigns the thermostat's IP address to another device, or after a power flicker that caused the Wi-Fi module to hang while the main processor remained active. You can confirm this by checking the Wi-Fi icon on the T6 Pro display; if it is flashing or shows a strike-through, the local connection is dead. If the icon looks solid but the app says offline, the issue is a "ghost connection" where the device thinks it is online but the server is rejecting the packets.

How to Fix It

To force a hard resync between the T6 Pro and the cloud servers, follow this precise sequence to clear the network cache and re-establish the handshake:

1. Perform a power cycle on your network hardware. Unplug your router and modem from the wall for exactly 30 seconds. Plug them back in and wait 5 full minutes for the 2.4GHz band to fully stabilize.
2. At the T6 Pro wall unit, press and hold the Menu button for 5 seconds.
3. Use the arrows to navigate to Wi-Fi Setup and press Select.
4. If the screen displays "Connected," navigate to Advanced Options and select Reset Wi-Fi. This wipes the stored SSID and password, forcing the device to request a new IP address from the router.
5. Once the Wi-Fi is reset, select Connect. The thermostat will enter "Setup Mode."
6. Open the Resideo app on your smartphone. Do not delete the old device; instead, go to the device settings and select Update Wi-Fi Settings.
7. Follow the on-screen prompts to scan for the T6 Pro. When prompted, enter your 2.4GHz network credentials (ensure you are not attempting to connect to a 5GHz band, as the T6 Pro hardware is incompatible with 5GHz).
8. Wait for the "Success" message on the wall unit before closing the app.

If That Didn't Work

• Assign a Static IP: Log into your router's admin panel (usually 192.168.1.1 or 192.168.0.1) and find the DHCP reservation table. Assign a permanent static IP address to the MAC address of the T6 Pro. This prevents the "Offline" error caused by IP address conflicts when the lease expires.
• Disable "Airtime Fairness": If you have a high-end ASUS or TP-Link router, disable the "Airtime Fairness" or "Smart Connect" feature in the wireless settings. These features often kick low-bandwidth IoT devices like the T6 Pro off the network to prioritize faster devices, causing the app to show the device as offline.
• Check C-Wire Voltage: Use a multimeter to test the voltage between the R and C wires. If the voltage drops below 24VAC, the Wi-Fi chip may shut down to save power for the relay switches, even though the screen stays on. If voltage is low, you may need a 24V transformer or a C-wire adapter.
• Factory Reset: If the software is frozen, go to Menu > Advanced Setup and find the factory reset option. Note that this will erase your heating/cooling schedules and you will have to re-configure the equipment settings (Heat Pump vs. Conventional) from scratch.

Heads Up

• The T6 Pro exclusively supports 2.4GHz Wi-Fi. If your router uses "Band Steering" (merging 2.4GHz and 5GHz into one name), the thermostat may intermittently drop. Creating a dedicated 2.4GHz Guest Network is the most stable long-term solution.
• Avoid placing large metal objects or mirrors directly in front of the thermostat, as these can degrade the Wi-Fi signal strength and cause frequent "Offline" timeouts.
• Ensure your router firmware is updated. Outdated WPA2 security protocols can sometimes cause the T6 Pro to fail the authentication handshake during a reboot.
• If you change your Wi-Fi password, you must perform the "Reset Wi-Fi" process on the wall unit first; you cannot update the password solely through the app.