The Host Diagnostics (HostDiag) screen shows various system information. It has been present on LG TVs since long before webOS. While it generally isn't very useful, it could come in handy in certain situations.
In general, to open HostDiag you must highlight "Channel Tuning" in the menu and press the "1" button 5 times. The menu can be opened with the menu button on the remote, but the steps after that vary a bit across webOS versions. Note that
On webOS 6 (2021 models), to reach Channel Tuning you must open the menu, then choose General (the gear icon) followed by Channels. While Channel Tuning is highlighted (as shown below), press the "1" button on the remote five times in relatively quick succession.
Channel Tuning highlighted on webOS 6.
On webOS 4.0 (2018 models), after opening the menu, choose "All Settings" (the gray circle with three vertical dots, becoming a pink circle with a gear when it has focus) at the bottom of the screen. "Channels" should be the third category down and has a satellite dish icon. Pressing right while Channels is selected should highlight Channel Tuning (as shown below), the first item in the Channels menu. Once that is done, press the "1" button on the remote five times in a row.
Channel Tuning highlighted on webOS 4.0.
Information displayed on the HostDiag screen includes the TV model number, firmware version, and OS version. There is also status information for the tuner ("FAT Status"), Wi-Fi adapter, and HDMI input. On webOS 6, HostDiag has sub-screens for Wi-Fi and HDMI that are not present on the webOS 4.0 version.
The HostDiag screen on webOS 4.0.
The main HostDiag screen on webOS 6.
The HostDiag HDMI screen on webOS 6.
The HostDiag Wi-Fi screen on webOS 6 (Wi-Fi not in use).