I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
MacBook Pro 14″, macOS 26.0
Macbook Pro M4 Max 128GB RAM, OS 26.0.1. -> Kensington TB4 Docking Station, LG 49W-L59C Monitor (attached via USBC to Docking Station).
With me everything Thunderbolt (I have a Thunderbolt 4 Dock by Kensington that has been the MBP's primary companion since I brought it home) takes random dumps flapping disconnect/connect after upgrading to OS 26. As others say, undesired behavior stops if you reboot with the TB4 dock connected, or if you leave it alone for a while with the lid open and just wait it out for a *very* long time (1/2h or more, every flap seemingly "lasting longer" on until I feel I can finally close the lid). Open or closed lid doesn't matter (though closed lid goes to sleep eventually).
It flaps all my TB peripherals back and forth as well, so it's the connection dock-Macbook, not USBC monitor-dock. It's insane and it was all working perfectly before the upgrade.
Setup works well with other devices including an non-upgraded Macbook Air M2 I also own.
You can claim "now we perfect video with no dropped frames" all you want, but graceful degradation on regular usage really is table steaks expected behavior in 2025 (and that it "eventually fixes itself" with lid open suggests a software bug, not cables). Submitted a bug and everyone who can reproduce should as well.
I write software (but not Kernel stuff). Here's a log around one of the flip-flaps (filtered by IOThunderboltSwitch) in case interested Apple devs see this.
default 16:31:54.565680-0700 kernel 130796819157us IOThunderboltSwitch(0@1)::finalize - terminate device: Kensington TB4 Docking Station (047d:809b:01) (8087:0b26:03)
default 16:31:55.780139-0700 kernel 130798033610us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 2 plug = 1
default 16:31:55.780402-0700 kernel 130798033873us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 1 plug = 1
default 16:31:55.895400-0700 kernel 130798148869us IOThunderboltSwitch(0@1)::syncTargetAndNegotiatedWidth - port (0@0:2) - bonding took 0 ms
default 16:31:55.895863-0700 kernel 130798149332us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 2 plug = 0
default 16:31:55.907162-0700 kernel 130798160632us IOThunderboltSwitch(0@1)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x1 port = 2 plug = 0
default 16:31:55.960740-0700 kernel 130798214209us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:55.961412-0700 kernel 130798214881us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:55.993346-0700 kernel 130798246815us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:55.994040-0700 kernel 130798247510us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:56.004702-0700 kernel Sandbox: ThunderboltAccessoryUpdaterServi(1256) allow iokit-get-properties iokit-class:IOThunderboltSwitchType7 property:Router ID
default 16:31:56.144109-0700 kernel 130798397578us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:56.144807-0700 kernel 130798398275us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:56.594343-0700 kernel 130798847808us IOThunderboltSwitch(0@1)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x1 port = 13 plug = 1
default 16:31:56.691675-0700 kernel 130798945140us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:56.692414-0700 kernel 130798945879us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
While cable changes may work for others, it hasn't been the case for me. My cables are new T5/80Gb cables (I already paid for a maxed out M4 Max, what's a TB cable?)
What sucks is that every other laptop including my Linux laptops of a huge range of versions and generations all work and behave beautifully with this exact setup... but what's by far the most expensive machine I've ever bought, decides to take random dumps now. It's just frustrating and makes me feel like I wasted my money here (when I know a crappy bus implementation when I use one; I mean, I have windows machines too).
I really hope they fix this. Soon. It's not okay since it hits me at least 2x/day as I move around with my laptop as one does.
Good luck everyone, hope any of this helps.
that Samsung CH890 display is a 3440 by 1440 display, like a 4K display with the bottom half missing. it supports 8 bits/color and refresh rates up to 100 Hz.
interfaces include:
2x HDMI 2.0
1x displayport 1.2
1x USB-C providing up to 65W power
to connect via USB-C, your USB-C cable must be certified and show the USB-SuperSpeed PLUS logo or the USB-20 logo, and be ONE meter or shorter. longer cables could cause transmission errors, which would cause the display to drop out.
over DisplayPort or USB-C, provided the cables are adequate spec and not too long, standard 4K 8 bits/color displays support up to 75 Hz without compression, and likely higher with Display Stream Compression.
I'm having some massive problems after updates too. I have a wavlink wl-ug63pd25 dock that I was using with a quad monitor setup (with 1 monitor plugged directly into the m4 macbook's hdmi port) and this worked for all day every day work usage for months. As soon as I upgraded to 15.7 the dock stopped being able to connect. I tried going up to Tahoe to see if that fixed it, same behavior. If I turn off the setting that automatically allows USB connections when the Mac Book is unlocked, then I get prompted every 15 seconds or so to allow the connection, and then the prompt disappears before I have a chance to click the button to allow. I've tried generating logs with this command:
log stream --style syslog --predicate '(subsystem == "com.apple.usb") || (eventMessage CONTAINS[c] "USB") || (eventMessage CONTAINS[c] "XHC")'
It's a bit cryptic, but it basically just shows the device constantly disconnecting and reconnecting. I have a second dock of the same model, and tried swapping cables and docks and did really everything you can think of to troubleshoot this type of thing at least 10 times. I tried playing with the displaylink driver versions too, but I don't think the dock is even staying connected long enough for displaylink to initialize. I also tried uninstalling displaylink altogether and just trying to use the usb and rj-45 ports on the dock, which should work without any driver/software installation, that doesn't work either.
One thing that does halfway work, is using a C-to-A cable and then converting it back from A-to-C with an adapter at the laptop port side. But this results in severely reduced throughput and I can't use all 3 monitors anymore.
I think it's extremely obvious at this point that something changed with the USB port management software in these recent updates. A lot of people are reporting issues, and for everyone reporting issues there's probably at least 10 others impacted who don't have time to tshoot this kind of stuff. It would be great if we could at least get some acknowledgment from Apple on this and explain what might have changed, and if we can expect some kind of fix in 26.1 or what the deal is.
I'm going to go ahead and order a new cable just to test with, even though I doubt that the wavlink one provided with the dock is any worse than what I can find from a third party. I've lost a lot of time and productivity due to this issue and I'm regretting my choice to "think different" and give apple a chance. I've been using quad monitor setups on Windows for decades and have NEVER had issues like this. It's ridiculous that this is what you get when you drop $1700 on a so-called "high end" device.
<< wavlink wl-ug63pd25 dock >>
The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. This requires a hardware rasterizer/display-generator for each fully-accelerated display, supported by Huge memory bandwidth to refresh each display 60 or more times a second.
Because of the bandwidth required for modern displays, Apple DOES NOT support multiple Hardware-Accelerated displays on ONE USB-C port. On a genuine Thunderbolt port connected to a ThunderBolt Dock, you can support as many as two, provided bandwidth is available for both.
The dock you describe supports ONE fully hardware-accelerated display. Additional display are supported using DisplayLink.
When you upgrade MacOS, you may also need to upgrade the DisplayLink Drivers. These most recent version available on the Wavlink web site is version 13, released 28 July 2025. that may be the lasted one compatible with the Wavelink Dock. But that does not appear to be the current version, 14, provided by Synaptics, the developer of DisplayLink.
You should contact Wavlink for guidance.
https://www.synaptics.com/products/displaylink-graphics/downloads/macos
(continued)
Change thunderbolt mode not working for me, but what works:
May be some wrong setting for Thunderbolt was stored on Mac 🤷
hlsu wrote:
I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
Nice to see things are working smoothly...
I would shut down and restart more than once to sort glitches on an upgraded macOS.
ref: Connect one or more external displays with your Mac - Apple Support
With all due respect this has absolutely nothing to do with USB-C cables, but it is a buggy release from Apple. According to the Mac User Guide there are absolutely no changes in the requirements for thunderbolt / usb-c cables upgrading from MacOS 15 to MacOS 26.
Use USB-C cables with Mac - Apple Support
Stop sending people on a wild goose chase for changing their USB cables. It worked before the upgrade, but stopped working after the upgrade - it has absolutely nothing to do with the cable.
DisplayLink technology creates a "fake" display buffer in RAM, sends the data out over a slower interface to a stunt box with DisplayLink custom chips that put that data back onto a "legacy" interface. It is not a true "accelerated" display, and it can suffer from lagging. Just adding the DisplayLink Driver is not adequate to get a picture -- you need a DisplayLink "stunt-box" or a Dock that includes DisplayLink chips.
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It may be acceptable for a second display showing slow-to-change data such as computer program listings, stock quotes, or spreadsheets, but NOT for full motion Video, not for Video editing, and absolutely not for gaming. Mouse-tracking on that display can lag, and can make you feel queasy.
In a pinch, it may even play Internet videos (as one user put it) “without too many dropped frames".
If you are only doing program listings spreadsheets, stock quotes, and other slow to change data, DisplayLink can work for you, but requires you to make some strong compromises.
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It is really nice to know that you can use a DisplayLink display if you MUST have an additional display for some of the types of data I mentioned. But that is NOT the same as the computer supporting a second, built-in, Hardware-accelerated display.
These displays depend on DisplayLink software, and are at the whim of Apple when they make MacOS changes. There have been cases where MacOS changes completely disabled DisplayLink software, and it took some time for them to recover.
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I think the Big Surprise for a lot of Hub/Dock buyers is that they thought they were getting a "real" display, but actually got a DisplayLink "fake" Display. If you got what you expected in every case, I would not use such pejorative terms to describe DisplayLink.
Note: educated myself (was too late to edit my post) and looks like flipping the cord forced a fresh mapping of sorts (like what switching from sleep to wake, etc was supposed to accomplish)
Make sure the monitor is powered on before connecting it to the Mac. FWIW, I have a 16" M4 Pro MacBook Pro and I'm typing this on an external display, although mine is connected via Thunderbolt (TB cable to a Stone Pro TB3 dock then a TB cable to TB3-capable LG 5K:2K display).
DaveGarratt wrote:
I don't understand your line of questioning. How does this information help resolve the problem - the user says that the monitor worked before MacOs
More recent version of MacOS have been increasingly demanding that there be NO transition errors. If the display in question has a very slightly faulty USB connection at the display, then it is plausible that the increased demands of MacOS 26 push it from being marginal to failed.
One reason I spend a lot of time here is to learn stuff. I want to know what display has this "known history" of USB port issues. I would like to read about it and see if there might be ways to work around that problem
.
I have had my share of USB-C cable issues in the past, and swapping cables may be worth trying, but I don’t think this is the whole answer.
There is something not right with the update. I have 2 x USB-C displays connected directly to the ports on a Mac mini M4 pro. Worked perfectly before Tahoe. Now only one monitor will connect unless I switch the other one off and on again. EITHER monitor works when the other is switched off. When both are on, the second display does not connect. Doesn’t matter which one I power on, it works if the other is switched off. Swapped cables between the two monitors and switched each off separately. The problem does NOT follow the monitor or cable or port… Can’t help feeling this is a related issue.
Hey -- I'd give it a few days until the version of Mac OS 26.1 comes out. While I can't talk about anything here, it causes the system to delete my message, I think most of us will have much better luck once this update comes out this week. YMMV :-)
Connect it to the system directly, reduce the refresh rate to the lowest possible setting, connect it to the docking station again. If it then works you keep increasing the refresh rate till you hit the problem. Repeat and stop at the refresh rate that still worked.
For now I have to use a separate HDMI cable to connect to the external monitor, which was detected properly.
[Edited by Moderator]
it happens with Dell U3425WE monitors. With a standard 24"inch monitor I have no problems using the USB C ports
External Monitor Not Detected After macOS 26 Upgrade
