Honeywell has the manuals for most of their thermostats on their web site. Start here, https://customer.honeywell.com/en-US/Pages/Department.aspx?cat=HonECC%2520Catalog&category=Wi-Fi%2520and%2520wireless%2520models&catpath=184.108.40.206
, for the Smart thermostats.
The E43 error indicates that the thermostat can't communicate with the router. Make sure that the router can communicate with your WAN (wide area network). Check the power and that the Ethernet cable is plugged in and undamaged. Reboot the router (unplug it and wait before plugging in the power cord). If you recently changed the password or the router SSID (the name of the network that the unit uses in its communication), try resetting the internet connection. Similarly, if you have a new router, reset the connection. Make sure that you know the SSID, password and encryption for the router.
I've checked the directions to do this on the Focus Pro TH6320WF series. Press More on the thermostat for 3+ seconds until the screen displays two numbers and the button labels are Done, Back and Next. Press Next until the larger number on the left of the display shows 39. Use the Up/Down arrows to change this to 0. Press Done. Take another Wi-Fi enabled device (tablet, cell phone, laptop) and set it to look for other available networks. Find the network with Thermostat in the name and connect to that network temporarily. If you see a prompt about the type of network, call this a Home Network. Open your web browser and go to the thermostat's Wi-Fi set-up page. (If needed, navigate to http://192.168.1.1
.) Find your router's home network's name in the list of available networks. Press Rescan if the SSID is being broadcast but doesn't appear in the list. If the router does not broadcast the SSID, enter the router name in the text box. Enter the encryption mode for the router. Follow the on-screen directions and press the Connect button. Enter the router password when prompted for the information. Wait to see the Wi-Fi Setup Connection Success in the display. If it doesn't check if the password was correctly entered, including special characters.
I hope this helps. The manual for the Focus Pro thermostat is here: https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-2736EFS.pdf .