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For seemingly no reason, the macOS hotkey to control the brightness (control + F1/F2) of my LG UltraFine secondary display has stopped working.

Also, the Sound Preferences do not show LG UltraFine speakers anymore, so I cannot set the sound output to that display.

When I try to open LG Screen Manager, it quits immediately.

Can anyone else using LG UltraFine (23.7" 4K, specifically) confirm facing this issue? Any suggestions on how to fix this?

This Apple bug/personal oversight has become a nuisance on my home setup. Not only can I not control the display and sound on my UltraFine monitor through the iMac, I am unable to send the sound output from my iPad to the monitor. All this was earlier working without any problem.

When going to Apple > About This Mac > Displays, the UltraFine display shows up, implying that the monitor is being identified. But the sound and brightness controls are not working.

I have tried three different devices, an iMac, an iPad, a MBP. Neither worked, leading me to believe that the issue is with macOS/iPadOS.

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Summary

  1. Shutdown your device and completely take the plug out of the socket, to kill its power supply;
  2. Disconnect the power supply of your LG UltraFine monitor as well, by pulling the plug out of socket;
  3. Reconnect(/Power on) everything after ~15 seconds;
  4. The controls should come back to life.

Fully unplugging resets a few things in the UltraFine monitor as well as in the iMac.

Detailed

I spoke with Apple Support and the representative tried to help me solve this problem, but to no avail.

The only thing that session managed to get done was have my iMac restarted in Safe Mode. It did not solve the problem, but in the process my UltraFine's display's orientation changed. (I use it in portrait; It got reset to landscape.)

Since it changed something, try that first.

Next, while moving my desk I had to disconnect my UltraFine and iMac. When I plugged them back to another place, I started the iMac first without the display connected to it. Then I plugged in the power cord of UltraFine. And then I plugged the display's cable to iMac.

I understood that disconnecting an iMac off its power supply for ~15 seconds is how you reset its SMC. I think when I disconnected my machine to move it, I accidentally reset the SMC and that fixed the problem.

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To restart the brightness controller, in Terminal do sudo killall corebrightnessd (you'll be prompted for your password). Or Force Quit it a couple of times in Activity Monitor, where you can check that it has restarted by looking in "CPU Time" column which should now be near zero.

I use an old Apple Cinema Display and have to do this every so often.

For your other issues, you may wish to re-plug the monitor connection. This range of monitors have known issues with macOS.

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