How to Detect Displays on a Mac
Typically when an external display is connected to a Mac, it will automatically be detected and start working instantly, with the Mac either extending the desktop or mirroring the screen over to the newly attached display output. Sometimes that doesn’t happen however, and when a secondary screen is not detected automatically by the Mac, you’ll want to trigger the “Detect Display” function in Mac OS.
Like some other features in new versions of macOS and Mac OS X, the “Detect Displays” button is now hidden by default, no longer immediately visible in the Display Preferences of macOS Monterey, macOS Big Sur, macOS Mojave, Sierra, macOS High Sierra, OS X El Capitan, OS X Yosemite, and OS X Mavericks. This does not indicate a problem with the Mac or with the external screen you wish to use, you simply have to toggle the Option key to make the detect feature visible and then be able to run the detection as usual. This applies to secondary displays of all types, whether it’s an external monitor, AirPlay mirroring, AirDisplay, a projector, an HDMI connection to a TV, or whatever other additional screen you have attempted to connect to the Mac. If you’re having any issues either showing the feature or getting an external screen to show up properly, this is what you’ll want to do.
How to Use Detect Displays for External Screens on a Mac
With the secondary display to detect already connected to the Mac, do the following:
- Go to the Apple menu and choose System Preferences
- Choose the “Displays” panel
- Hold down the “Option” key to show the “Detect Displays” button – note that it replaces the ‘Gather Windows’ button
- Click on “Detect Displays” while holding down Option to use the function as intended
At this point the external screen should be found and work as usual, launching the secondary “Displays” window for that screen. Of course you’ll want to be sure the physical connection is secured for the external display, and if you’re having any issues check the cables to the LCD monitor, projector, or TV.
It should be pointed out that having the “Detect Displays” button not being visible in Mac OS is not an indicator of a problem or a bug, and it’s certainly not indicative of an issue with the output device, it’s simply hidden from casual use, presumably because most of the time the Mac usually has no issues finding and connecting to external screens. Nonetheless, sometimes you have to forcibly detect an external display, which is why this feature is a little curious to hide by default.
Using the “detect” feature should be the first troubleshooting step taken if you can’t find a video output device that is connected to a Mac, though more advanced methods may need to be taken if there are other issues, such as flickering or noisy displays, which may require an SMC reset to resolve.
Also, sometimes simply rebooting a Mac with the external display connected can resolve some unusual display and monitor issues.