trying to install and it's stuck on 'connecting to sensi' screen forever during wifi setup.

Diagnosis

The "Connecting to Sensi" hang occurs during the critical "handshake" phase of the provisioning process. At this stage, the ST75 thermostat creates its own temporary internal Wi-Fi access point (AP) to communicate directly with your smartphone. The app attempts to switch your phone's network connection from your home router to the thermostat's internal broadcast to pass along your Wi-Fi credentials. The failure happens when the smartphone's operating system (iOS or Android) detects that the Sensi AP does not have an active internet connection. Modern smartphones are designed to automatically disconnect from "no-internet" Wi-Fi networks and revert back to cellular data (LTE/5G) to maintain connectivity. When this happens, the Sensi app loses the bridge to the thermostat, leaving the screen stuck in a loop because the device is no longer "reachable" on the local network. This is often exacerbated by "Smart Network Switching" or "Adaptive Wi-Fi" settings on the phone.

How to Fix It

To force a stable connection and complete the setup, follow these precise steps:

1. Prepare the Phone: Go to your phone's Settings and disable "Cellular Data" entirely. This prevents the phone from jumping back to 5G/LTE when it realizes the Sensi AP has no internet.
2. Enable Permissions: Ensure Location Services (GPS) is set to "Always" or "While Using App." On Android, ensure "Precise Location" is toggled ON. Without this, the app cannot scan for the local SSID of the ST75.
3. Clear App State: Force-close the Sensi app. Go to your Wi-Fi settings and "Forget" any network named "Sensi-XXXX" if it appears in your saved list.
4. Reset the Thermostat: On the ST75 screen, navigate to the Menu > Settings > Reset and perform a "Network Reset" to clear any partial configurations.
5. Launch Setup: Open the Sensi app and start the "Add Device" process. When prompted to connect to the Sensi Wi-Fi, manually go to your phone's Wi-Fi settings and select the "Sensi-XXXX" network if the app doesn't do it automatically.
6. Complete Provisioning: Once the app confirms it is connected to the thermostat, enter your home Wi-Fi password. Once the "Success" message appears, you can re-enable your Cellular Data.

If That Didn't Work

• Split your SSID: The ST75 only supports 2.4GHz bands. If you have a "Smart" or "Mesh" router (like Eero, Google Nest, or Orbi) that combines 2.4GHz and 5GHz into one name, the thermostat may struggle to handshake. Log into your router settings and create a separate guest network specifically for 2.4GHz devices, then attempt the setup using that new SSID.
• Disable VPN/Ad-Blockers: Apps like NordVPN, ExpressVPN, or AdGuard create a virtual tunnel that intercepts local network traffic. This prevents the Sensi app from seeing the thermostat's local IP address. Disable these entirely in the system settings, not just the app, before restarting the process.
• Check Firewall/DNS: If you use a custom DNS (like Pi-hole) or a strict firewall, the ST75 may fail to reach the Emerson activation servers. Temporarily switch your router to Google DNS (8.8.8.8) or Cloudflare (1.1.1.1) to see if the connection completes.
• Power Cycle the HVAC: Turn off the breaker to your HVAC system for 30 seconds. This performs a hard reboot of the ST75 hardware, clearing any hung processes in the Wi-Fi chip that a software reset might miss.

Heads Up

• C-Wire Requirement: Ensure you have a dedicated C-wire (Common wire) connected. While some Sensi models claim to work without one, the ST75's touch screen and Wi-Fi radio draw significant power; without a C-wire, the Wi-Fi chip may intermittently brown out, causing connection drops.
• Signal Strength: If the thermostat is located far from the router, the "Connecting" screen may hang due to low RSSI. If you have a signal strength below -70dBm, consider a Wi-Fi extender placed halfway between the router and the thermostat.
• App Updates: Always ensure you are using the latest version of the Sensi app from the App Store or Play Store, as Emerson frequently releases patches for Android 13+ and iOS 16+ connectivity bugs.