Why is my second external display not working after macOS 15.5 update?

Settings > Desktop &Dock > Mission Control ...

displays have separate Spaces <--- turn that OFF if you want to use Extended Desktop

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there is one more crowd-sourced work-around, just posted nearby:

delete-

~/Library/Preferences/ByHost/com.apple.windowserver.displays.<hex_string>.plist and reboot

Does the display work in Safe Mode? If it does then the problem may be some third party software.

Also: there is no “reset NVRAM” in Apple Silicon macs.

Try starting your Mac in Safe Mode, with just the internal display.

You should be able to login, right?

Note: the change in "Displays have separate Spaces" should never cause this sort of trouble.

Unfortunately, it seems that there are some issues with your Mac, and the display problem may be merely a symptom of bigger issues.

If you haven't already, make sure to have an up to date backup!

Wait, what?

why are you not using a PREMIUM HDMI cable for one of the displays?

Does the dead display follow one particular adapter or the other? HDMI adapters get HOT and can burn out.

Same here. After updating to Sequoia 15.5 on my M3 MacBook Pro, the external monitor only works with the lid closed.

If I open the Mac, the monitor freezes or goes black.

Tried different cables, reset NVRAM, cleared display prefs — nothing helped.

Worked fine before the update. Definitely a Sequoia bug.

• The monitor works inside recovery/Startup Options, which proves it’s a system-level issue, not hardware

Be sure you have the right kind of processor to support the number of displays you are connecting.

Almost all the same rules we learned for the M1 and M2 and M3 mostly still apply to the M4 models. Display interfaces are generated deep inside the system on a chip. How many there are depends on the exact type of Processor:

M3 (plain) like the 13-in and 15-in supports up to ONE External fully hardware-accelerated external display.

Except the M3 MacBook Air models with 13 or 15-in display, (and certain M3 plain MacBook Pro models when running 14.6 or later) which can support a second External display instead of the built-in display when you close the cover on the built-in display. This may require MacOS 14.6 or later.

M4 (plain) supports up to Two fully hardware-accelerated External displays (three for Mac mini)

M4 PRO processor supports up to Two fully hardware-accelerated External displays. (three for Mac mini)

M4 MAX processor supports up to Four fully hardware-accelerated External displays.

M4 ULTRA processor supports up to Eight fully hardware-accelerated External displays

Tech Specs now available here:

MacBook Pro - Tech Specs - Apple

MacBook Pro - Tech Specs - Apple

are the displays mirrored?

does each display that is showing have its own menubar?

Have you checked that you definitely do NOT have a Menubar on each display after that sequoia update?

Have you found a solution? Same issue here... Got 2 external displays, as soon as I plug the second monitor, monitor #1 either turns black or freezes like a screenshot showing whatever it was displaying before monitor #2 is plugged. Not sure if it's worth mentioning but it's always monitor #1 that freezes or turns black no matter what order I plug in the cables.

Did try the following:

1. Deleting ~/Library/Preferences/ByHost/com.apple.windowserver.displays.<hex_string>.plist
2. Different cables
3. Boot in safe mode
4. Bang my head against the wall

Driving me crazy here.

I turned the setting off, restarted my MacBook and now every time I attempt to login it fails and crashes. My computer is bricked.