External displays not detected after macOS Tahoe 26.5 update

I did a bunch of combos with my 2 LG 32" 4K 3-4 year old monitors. and two different M5 pro max's, and two cables (USB4/60W/Thunderbolt 3 cable and a USB4/240W/Thunderbolt 5 cable).

Everything works flawlessly with my M1 Mac Pro, but the M5 Max's would not generally recognize the display, or their power over USB. An odd exception is that one of the M5's would work for the first time plug in after a reboot, and then stop recognizing the monitors, the other one just did not see the monitors.

Here's the annoying procedure I use now to repeatably get the LG monitors to work:

==> Using the Thunderbolt 5 cable (and MacOS 26.5.2 - I expect ..1 is the same)

1. Go to settings => displays

• Press "Options" to unhide the ""Detect Displays" button
• Click on the ""Detect Displays" button a few times
• unplug the USB cable
• replug in the USB cable to the same port

And the display would show, and power will flow as long as the cable stays plugged in, and the power to the monitor on. [note: unplugging within the first minute or so after my above procedure does not lose the monitor connection]

Results with the older USB 4 cable were less consistent.

Adding in my experiences for whatever it is worth,

I found an old Apple LED Cinema Display that no one was using at work so I grabbed it as part of my desk setup. At first, I was connecting to it with my personal 13" MacBook Air M4 but upgraded at work to a 15" MBA M4 so I am connecting to that. Part of the reason the LED Cinema Display was abandoned was because it is a mini-DVI device which is much more difficult to use with HDMI being ubiquitous. The LED Cinema Display is pretty big (with big bezels) for a 2560x1440 display but the image is still great so as a longtime Mac user, I wanted to use it on my desktop.

Firstly I have a few USB-C docks that have a mini-DV connection on them but generally the PD power is relatively low which would occasionally cause problems. At this point, I am using a IOGEAR GUD3C02 USB-C Ultra-Slim Dual Display Docking Station with Power Pass-through, which generally worked well with either of the MBAs although if the USB-C connection to the laptop got jostled, occasionally the picture on the display would go away requiring me to go through replugging connectors and rebooting to get the picture back - a bit of a hassle but it was something that I could deal with.

Since upgrading to macOS Tahoe 26, it has become a lot more problematic and it seems that it is something in the OS software that is the issue. When the issue comes up, it takes a lot more plugging and plugging and powering and repowering to get it back, and sometimes when I do get it back, it only recognizes the LED Cinema Display as a 1920x1080 monitor.

It appears as though there is something that gets hung up in system preferences for the external monitor. For me, the best way to get the monitor working again is to do a total shutdown, a power reset by holding down the power button for 10 seconds to clear system boot prefs, and then power up again. The latest time, when I powered up, it did come up as 1920, but when I went into the Displays control panel, I was able to see that all of the display resolutions were coming up, including the 2560 option.

As with many other posters here, the monitor connection seemed to work before Tahoe since while not 100%, when the monitor was not recognized, I could do reboots and reconnects to get the monitor up and running in less than 5 minutes. Since Tahoe, it has taken me a lot more steps to get the monitor working, sometimes wasting 15 -30 minutes and happens more often. It definitely appears that there is a bug somewhere that breaks the ability for Macs running MacOS26 to connect to older monitors.

In the end my conclusion was: OS26.5.1 is a bit choosy about what monitors it can drive. So, if you have an old monitor, and it doesn't work when plugged directly into the HDMI port, try using a USB3 to HDMI dongle (which will probably also give you extra USB ports as well - and if your setup is anything like mine, these will be useful) as the Mac will probably drive it quite happily, and the dongle will drive the (old) monitor. And if you have a newer monitor, it will probably work quite happily on the machine's HDMI port. I hope this is useful.

To the OP and ALL those Affected...

"External displays not detected after macOS Tahoe 26.5 update: Is anyone else having issues with external monitors after updating to macOS Tahoe 26.5 (25F71)? [...]I've also found a bunch of posts from other people having similar issues after updating to Tahoe 26.5, especially with external displays, docks, and clamshell mode. Just wondering if anyone has found a fix yet, or if this is looking like a Tahoe 26.5 bug?"

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No. I have not come across this issue.

So,...

A. Report this to Apple:

Do your Part: Report this to Apple. Apple may not get back to you directly, but the more feedback Apple receives on this, the more they will know what is going on, and what needs to be fixed in upcoming updates. You mention that you've seen many reports of this. Question is: "Just how many people have reported this to Apple?" So, to get the word out, report this to Apple. Apple will know nothing of your experience, if you do not report it to them.

1. Go Here: Feedback - macOS - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: that this all started after installing an update, and how you have troubleshot it thus far. Include the URL to this thread.
4. Proceed from there as necessary

B. Try Safe Mode:

Boot in Safe Mode, and see if this no longer occurs. While in Safe Mode, login items do not open at login, and certain caches are fixed. Restart normally, seeing if it then works.

C. Single-Out this User:

For troubleshooting purposes, create new administrator user, seeings if you still get this issue. If so, then it is a system-wide issue. If not, then it is an issue pertaining to this one user (meaning it is likely a misconfiguration).

Hi, I'd the same problem my "solution" has been reinstall MacOS from the recover service.

I write "solution" because now my screen works throught the hub but not in native HDMI I've Macbook M4 Max.

I think that apple in next update, will resolve it.

PD: I tested the same screen in macbook m1 with 26.5.1 and it works perfect, so I think that yes, the problem is on 26.5.2 ...

to everyone suffering from this problem.

PROVIDED your cables, adapters and Docks are all appropriately capable and appropriately short (details as described in my posts above) there is one more thing that MIGHT get you different results:

As a last resort, For MacOS 12 Monterey and some later versions, some users got relief by deleting some preferences files, then Restarting.

To access the folder using Finder for the system-wide preference

Copy and paste this string into:

Finder > Go menu > Go to Folder

/Library/Preferences

then find and delete this file:

/Library/Preferences/com.apple.windowserver

-AND- To access the folder for your-user preference

Copy and paste this string into:

Finder > Go menu > Go to Folder

~/Library/Preferences

then find and delete whichever of these two files you find:

~/Library/Preferences/com.apple.windowserver.<UUID>.plist -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist

-- where [~] tilde is an active abbreviation for "current user"

-- and <UUID> is a string of hexadecimal numbers unique to your system, grouped as 8-4-4-4-8 hexadecimal digits

Restart REQUIRED after these deletions.

--------

recent experience by other users suggest the files you might find at:

~/Library/Preferences

could be any of:

then find and delete whichever of these files you find:

~/Library/Preferences/com.apple.windowserver.<UUID>.plist   -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist   -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.displays<UUID>.plist 

C4rp20 wrote:

Seeing similar on DisplayLink hub, also the MS Teams app has started losing connection with external camera(s) via the same hub with picture dropping out and freezing during calls (tested across multiple camera devices).

To get the attention you deserve, PLEASE start a NEW discussion.

You likely do not have "exactly the same problem" and need to explain from the beginning. This discussion is already crowded with different problems posted by different users with different hardware, different versions of MacOS, and different displays. 

Starting a new discussion will allow Readers to think hard about your exact situation and address the exact circumstances you are facing.

Please be certain to include the make%model of each display the exact model of any docking devices, and exactly what cables are connected what-to-what.

https://discussions.apple.com/post/question

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

Cables 'shipped in the box' with many displays can be "lowest bidder" cables that are not up to high-end display use. Cables with no markings 'found' behind your TV set are unlikely to be good enough.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

This worked for me on a 2021 MacBook Pro (M1 Pro) running Tahoe 26.5.1 with a Huion Kamvas Pro 13 (GT-133). I moved the WindowServer display preference files out of the way, restarted, and macOS recreated them. My external display was detected immediately afterward.

Interesting. The displays are a Dell SE2417HGX, and a Acer which shows up as DP2HDMI 13850 (but I think that's the chip on the dongle). With the Dell plugged in via the USB port and dongle) and the Acer plugged directly to the HDMI port, only one display works (the Dell). With them plugged in the other way round, both work. I haven't the time or energy to try them the other way round again... just happy that it's working again. I suspect the Acer is the culprit, in some sense, as it doesn't seem to provide its own colour profile, and I think the monitor is quite old. But at least it's working now.

I don't really understand why all of your replies on this post are so combative. There are only 15 posts on this thread, it's not that crowded.

You haven't really added any constructive suggestions other than to insinuate others aren't living up to Apple's standards or that it's somehow their fault. The problem with that thinking is our setups were working before the upgrade & now they aren't.

I've spent 2 days troubleshooting and after going through 3 different docks & 5 different cables I realized the only new change that occurred right before I started having the issue, was the update to 26.5.2. I'm not ready to say it's definitely the cause yet, but it's starting to point that way.

Looks like 26.5.1 bug (both displays working earlier). Reported to Apple. (Mac mini M1, 2020).

Seeing similar on DisplayLink hub, also the MS Teams app has started losing connection with external camera(s) via the same hub with picture dropping out and freezing during calls (tested across multiple camera devices).

what make& model is each display?

exactly what cables adapters and docks (including model numbers on Docks) are being used to make the connection?

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video. Folks really are producing commercial feature films on Apple equipment, so It MUST meet the highest standards. That can not tolerate any dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or Transmit errors due to low-spec or overly long cables.  More recent versions of MacOS are more demanding than any other systems.

<< I don't really understand why all of your replies on this post are so combative. >>

I really don't know how you came to that conclusion.

Connecting displays can be quite complex, and the devil is in the details.

• some displays can not work reliably on certain connections. For example a display higher than 4K is unlikely to work well on a USB-only adapter or connection. Some Macs can not support higher resolution HDMI connections.

• Cables and connections are very important, including using appropriate spec cables that are not too long.

• Docks adapters, and switches make things even more complicated.

• MacOS has traditionally only set display resolutions after a query to the display, sent only at certain times. Failing to understand that leaves your perfectly good display setup dark.

• Increasing versions of MacOS have decreasing tolerance for displays whose setup makes transmit errors, sometimes due to cables too long.

It's complicated. And each case is different.

Readers here are all Volunteers. I start by reading what a user is willing to share, often ask for more information, then go on to other things. Right now I am trying to fix the siding on my house after blowing insulation into the walls.

Later, I try to revisit the discussion. If lots of people say, "me too" without providing details of their exact setup, it takes a great deal of time and energy to sort out which problem is which. So I try to ask Users to start a new discussion.