the app says my thermostat is offline but the screen on the wall looks fine and is working.

Diagnosis

This issue is a classic "state mismatch" where the local hardware is functioning correctly—meaning the thermostat is still controlling your HVAC system via the physical wires—but the network handshake between the RTH6580WF's internal Wi-Fi chip and the Honeywell Total Connect Comfort (TCC) cloud servers has been severed. Because the thermostat operates on a "local-first" logic, it will continue to run your heat or AC based on the set point on the wall even if it cannot "phone home" to the app. The root cause is typically a DHCP lease expiration, a 2.4GHz frequency interference, or a router firmware update that dropped the device's static assignment. You can confirm this by checking the Wi-Fi icon on the thermostat screen. If the icon is blinking, the device has lost its connection to your local router. If the icon is solid but the app still says "Offline," the device is connected to your router, but the router is blocking the specific ports required for the TCC cloud communication, or the cloud server is experiencing a timeout with your specific MAC address.

How to Fix It

To restore the cloud connection and get the app back online, follow these precise steps to power-cycle the network handshake:

1. Verify Local Connectivity: Press the MENU button on the thermostat. Navigate to Wi-Fi. If the screen displays "Disconnected" or the Wi-Fi icon is flashing, the hardware has dropped the signal.
2. Hard Reset the Network Module: Do not just restart the router. Go to your electrical panel and flip the breaker for your HVAC system (Furnace/Air Handler) OFF for exactly 30 seconds. This forces the RTH6580WF to reboot its internal wireless card and clear its temporary cache. Flip the breaker back ON.
3. Refresh the Router: While the thermostat is rebooting, unplug your router from the wall power. Wait 60 seconds. Plug it back in and wait for the 2.4GHz band to fully stabilize (usually 3-5 minutes).
4. Force Re-association: On the thermostat, press MENU > Wi-Fi. If it does not automatically reconnect within 2 minutes, select Setup and re-enter your Wi-Fi password. Ensure you are connecting to a 2.4GHz SSID; this model cannot communicate on 5GHz bands.
5. App Sync: Close the Total Connect Comfort app completely (swipe it away from your recent apps) and relaunch it to force a new session request to the server.

If That Didn't Work

• Assign a Static IP Address: Access your router's admin panel (usually 192.168.1.1 or 192.168.0.1). Find the RTH6580WF in the connected devices list and assign it a "Static IP" or "DHCP Reservation." This prevents the router from changing the thermostat's address, which is the primary cause of intermittent "Offline" errors in this model.
• Change the Wi-Fi Channel: If you live in a crowded neighborhood, signal interference on Channel 6 or 11 can knock the thermostat offline. Change your 2.4GHz wireless channel to "Auto" or manually set it to Channel 1 to reduce congestion.
• Update DNS Settings: Some ISP DNS servers struggle with Honeywell's cloud routing. In your router settings, change your Primary DNS to Google's DNS (8.8.8.8) and Secondary to (8.8.4.4). This often resolves "Solid Icon but App Offline" issues immediately.
• Factory Reset and Re-register: As a last resort, press MENU, navigate to Reset, and select Factory Reset. You will need to remove the device from your TCC account via the web portal and add it as a new device using the MAC ID found on the back of the thermostat faceplate.

Heads Up

• C-Wire Requirement: Ensure you have a dedicated C-wire (Common wire) installed. If you are using a "power extender" or relying on power-stealing, the Wi-Fi chip may shut down during high-demand HVAC cycles to save power, causing the app to go offline.
• Signal Strength: If your router is more than 30 feet away or separated by more than two walls, the RTH6580WF may struggle. Consider a Wi-Fi extender placed halfway between the router and the thermostat.
• Security Protocols: This thermostat works best with WPA2-AES security. If your router is set to WPA3 only, the thermostat will fail to maintain a stable connection.