If your Mac doesn’t recognise an external display
If your Mac doesn’t recognise that a separate display is connected, you can try to detect it in System Settings or try one of the other troubleshooting tips below.
On your Mac, choose Apple menu > System Settings, then click Displays in the sidebar. (You may need to scroll down.)
Press and hold the Option key to make the Detect Displays button appear in place of the Night Shift button, then click Detect Displays.
If your Mac still doesn’t recognise the attached display, try any of the following:
Unplug the display from your Mac, then plug it back in.
Make sure the cable you’re using is securely connected to the correct port.
Make sure you’re using the correct cable to connect your display.
Tip: To get more detailed information about the type of video display your Mac supports, check the tech specs for your Mac: Choose Apple menu > System Settings, then choose Help in the menu bar. Choose [your Mac’s name] Specifications, then scroll down to Display Support or Video Support (depending on your Mac).
Make sure you’re using the correct adapter, if needed. See the Apple Support article Adapters for the Thunderbolt 4, Thunderbolt 3 or USB-C port on your Mac.
If you’re using a third-party display, check the included documentation to ensure you’re connecting the display according to the manufacturer’s guidelines.