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.
<< Just to add to the case to possibly help the devs work on a solution. >>
Readers here are other users like you. Most Apple employees are prohibited form participating here. No one at Apple is working on a solution unless YOU and others make a report You can make an informal report here:
You can also contact telephone support, and work with them. If no resolution, you can ask them to start a formal, tracked Bug Report.
I won't go into details, but for me, the problem 100% was the cable.
Everything was working perfectly on Friday night.
I updated to MacOS26 over the weekend (went smooth).
This morning at the office, I plugged in all three HP 27er external monitors and one kept throwing an "Input signal out of range" error.
I factory reset that monitor.
I tested the "bad" & "good" cables in numerous scenarios. The error message always came back to one specific cable that was causing the issue.
Installing a new cable brought all 3 monitors back to life.
It had to be this one HDMI to USB-C cable. The odd thing is, a twin cable to this one still works perfectly. Go figure.
Thanks to everyone for your posts, very helpful.
[Edited by Moderator]
I had roughly the same issue, I have 2 displays plugged in directly into my Mac M1.
I go from USBC on the External Display to USBC on my MAC M1, On Tahoe 26 everything worked fine. The Displays would come on with the Laptop open and would go off momentarily when I closed the laptop and come right back on. The laptop was also charging simultaneously thru the 2 USBC Cables.
Once I upgraded to Tahoe 26.0.1 the displays stopped working when the Laptop was closed but would work when the laptop was open. After seeing you post I checked and saw the Laptop was no longer charging from the USB Cables so I plugged in the Mag Safe Adapter and the displays started working working and charging with the Laptop closed. Interesting that Your system required you to Unplug power and mine required me to Plug in power. I know I am not using a Dock but these are Tahoe Issues. I tried my USBC cable on the same setup with my M4 and it worked fine.
"Now that I can reproduce it, I'm writing up a bug report for Apple. Good luck, everyone!"
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As a followup, did you report this bug and the fix directly to Apple? Or did you just post a report on these forums? Apple rarely reads these forums on their own time and will. So, if not reported directly to Apple, then do so, yourself.
Report this Bug and this Solution to Apple:
Do your Part: Provide Apple with feedback on this, letting them know about this issue. Apple may not get back to you directly, but the more feedback they receive on this, the more they will know what is occurring, and what bug fixes to include in updates to come. Apple will know nothing of this, if you don't report it to them.
Kind of unbelievably, flipping the cable also worked for me. Just literally flipped it. None of the other solutions were working so tried this one before replacing the cable and it worked. USB C to USB C cable. No dock in between, just direct from macbook pro to dell external monitor.
Yes, my monitor also has a TB4 port. Before upgrading to 26.1, a USB-C Alt cable worked perfectly fine, but after that, it doesn't work. Switching to a TB4 cable works.
Using another cable "fixes" the issues. Nevertheless it is kind of strange that the problem only appears on macOS 26. I tried a different MacBook with macOs 15 and it worked flawlessly.
Hi All,
I was facing the same problem.
I connected the monitor with an HDMI cable to my MacBook, restarted, and then simultaneously connected the USB-C cable. When I disconnected the HDMI, the problem was no longer there.
It seems like a 'first-detection' problem.
This fixed it for me:
This worked for me (M2 MacBook Air) as well, but only if I did things in this order:
* Set Thunderbolt mode on the display to "Standard"
* Plug in MacBook, let it glitch out
* Switch Thunderbolt mode to "Optimized"
This worked for me (M2 MacBook Air) as well, but only if I did things in this order:
* Set Thunderbolt mode on the display to "Standard"
* Plug in MacBook, let it glitch out
* Switch Thunderbolt mode to "Optimized"
Hi! The only thing that worked for me was flipping the cord so each end is connected to the opposite port
Think it forces a re-mapping of sorts
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.
<< 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. >>
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.
More recent versions of MacOS has ben ever more demanding about transmit errors, which can be caused by cable that are not quite high enough specs.
If your cables are already up to that spec, then all you will need to do is is INSPECT your cables for those markings, and when found, THEN you can eliminate that as a possible issue.
Some readers have reported that bringing the cables up to the required spec solved their problem, so I will continue to post that advice.
I had this same issue with a Mac Mini M4 after upgrading to 26.1 from 26.0.1. I have 3 monitors and the 3rd one (at 3440x1440 resolution) went blank. That monitor was connected via a HDMI cable that had a HDMI to USB-C adapter on it. Switched that out to a Thunderbolt cable and that didn't fully resolve the issue. There was a slight improvement: the 3rd monitor went from not being recognized at all by the Mini to the monitor being recognized but no signal reaching the monitor. Finally I found this page: How many displays can be connected to Mac mini - Apple Support. and discovered this tip: "Connect the display with the highest resolution first.". So I moved this blank monitor to the first USB port in the back with the USB-C cable (didn't work) and finally the HDMI with HDMI/USB-C adapter, and it worked. Not sure why this was an issue, but I'm happy it is resolved. Good luck!
"External Monitor Not Detected After macOS 26 Upgrade: 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."
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Verify the Connection:
Just curious: How are you verifying this connection? To, see if the monitor is even detected,...
External Monitor Not Detected After macOS 26 Upgrade
