Quick Fix: Keyboard Brightness Controls on macOS (external Apple Display)

I recently had an issue where the keyboard controls to adjust monitor brightness weren’t functioning on my Apple Macbook Pro running macOS Sierra. The Displays pane in System Preferences showed no brightness adjustment despite showing the correct monitor visual and name (LED Cinema Display, in my case).

It turns out that Apple uses the USB connection to communicate with these monitors. That connection had, for one reason or another, failed to be initialized or was erroneously terminated. The fix was simple: unplug the USB cable that goes from the monitor to the computer from the USB port on the computer, then reinsert it. No restarting necessary.

After this fix, the Displays pane went back to normal showing the brightness adjustment:

macOS Sierra Displays preference pane showing brightness adjustment

 
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