thermostat keeps losing wifi connection every few days, have to reboot it to reconnect.

Diagnosis

The Emerson Sensi Touch ST75 utilizes a low-power Wi-Fi chipset designed for energy efficiency, but it often struggles with "DHCP Lease" timeouts and modern router "Band Steering" protocols. When the thermostat requests a renewed IP address from your router, a handshake failure occurs—often because the router attempts to push the device from the 2.4GHz band to the 5GHz band. Since the ST75 only supports 2.4GHz, the connection drops entirely and the device enters a "zombie" state where it cannot reconnect without a hard power cycle.

Additionally, this instability is frequently caused by WPA3 security protocols. Many modern routers (Wi-Fi 6/6E) use a "Mixed Mode" (WPA2/WPA3). The Sensi Touch often misinterprets the WPA3 beacon as a corrupted packet, causing the wireless radio to crash. You can confirm this is the issue if your thermostat remains connected for exactly 24 or 48 hours (the standard DHCP lease time) before dropping, or if the "Wi-Fi" icon on the display disappears while other devices in the same room remain online.

How to Fix It

To permanently stabilize the connection, you must move the thermostat from a dynamic environment to a static, dedicated configuration. Follow these exact steps:

1. Separate Your Wi-Fi Bands: Log into your router's admin panel (usually 192.168.1.1 or 192.168.0.1). Locate the Wireless Settings. Disable "Smart Connect" or "Band Steering." Create two distinct SSIDs: one named "Home_WiFi_2.4" and one named "Home_WiFi_5." Connect the Sensi Touch specifically to the 2.4GHz network.
2. Force WPA2 Security: In the security settings for the 2.4GHz band, change the authentication mode from "WPA2/WPA3 Mixed" to "WPA2-PSK (AES)." This prevents the radio crash caused by WPA3 beacons.
3. Assign a Static IP/DHCP Reservation: Find the "DHCP Client List" or "Static IP" section of your router. Locate the Sensi Touch by its MAC address (found on the back of the device or in the Sensi app under Device Info). Assign it a permanent IP address (e.g., 192.168.1.50). This eliminates the "Lease Timeout" disconnects.
4. Disable Interference Settings: Ensure that "AP Isolation," "Guest Network Isolation," and "IGMP Snooping" are all toggled to OFF. These features prevent the thermostat from communicating with the Sensi cloud server.
5. Hard Reset and Re-pair: Remove the thermostat faceplate from the wall for 30 seconds to clear the volatile memory. Snap it back on and re-configure the Wi-Fi settings via the Sensi app using the new 2.4GHz SSID.

If That Didn't Work

If the connection still drops after the steps above, try these advanced alternatives:

• Install a Dedicated 2.4GHz Access Point: If your router is more than 30 feet away or separated by dense walls (brick/concrete), the ST75's small internal antenna may be failing to maintain the signal. Install a cheap 2.4GHz-only travel router or access point within 10 feet of the thermostat to act as a bridge.
• Change the Wi-Fi Channel: Log into your router and change the 2.4GHz channel from "Auto" to a fixed channel—specifically 1, 6, or 11. Overlapping channels from neighbors can cause the ST75 to drop the connection during noise spikes.
• Check C-Wire Voltage: Use a multimeter to test the voltage between the R and C wires. If the voltage is dipping below 24VAC, the Wi-Fi radio may shut down to prioritize the HVAC control circuitry. If you lack a C-wire and are using a "Power Extender Kit," ensure the connections are tight and not oxidized.
• DNS Server Swap: In your router's WAN settings, change the DNS servers from your ISP's defaults to Google DNS (8.8.8.8 and 8.8.4.4). Sometimes the Sensi fails to reconnect because it cannot resolve the Sensi cloud URL via the ISP's DNS.

Heads Up

• Avoid 5GHz: Never attempt to connect the ST75 to a 5GHz network; it is physically incapable of seeing these bands and will result in a "Connection Failed" loop.
• Firmware Updates: Always check the Sensi app for firmware updates immediately after a successful reconnection. Do not power cycle the device during an update, as this can brick the Wi-Fi module.
• C-Wire Necessity: While some Sensi models claim to work without a C-wire, the Touch ST75 requires a consistent 24V power source for the Wi-Fi radio to stay active. Using a "power stealing" method often leads to the exact intermittent drops you are experiencing.
• Router Reboots: If you have a scheduled router reboot (common in some mesh systems), the Sensi may struggle to re-authenticate. Disable scheduled reboots or add the Sensi's MAC address to the router's priority list.