External Monitor Not Detected After macOS 26 Upgrade

Hey everyone, I've also just experienced this issue after upgrading to macOS 26. So a few things. I am nearly certain this is not due to the cable. Everything was working as it should before the upgrade. The issue appears to occur with USB-C connected monitors, particularly when using a docking station.

Here's how I reproduce the issue. When I use my Mabook in clamshell mode, it functions as expected. Now, if I unplug the dock and then use the laptop with nothing else connected, it works. But when I replug the Dock into the computer, that's where I get issues.

A few things I've tested: unplugging and replugging the monitors - no go.

Unplugging the Dock and replugging that in, no go. Plugging in the USB-C monitor directly to my MacBook Pro M1 Max, and still no go.

** In that Dock, I have an external USB HD connected, as well as other USB components, and if the monitors don't come into play, the drives do not show up either, so nothing is recongnized.

A few things that work, and a tentative solution until Apple resolves this issue. Plugging the monitor directly into the MacBook Pro M1's HDMI port works flawlessly. Additionally, for me, rebooting the Mac resolves the issue until I perform the above step, which is unplugging, which I do because I work at different locations.

I'm not an Engineer, nor do I play one on the Internet, but somehow the signal that checks if something is connected to USB after waking from sleep has a hiccup in it. The issue only occurs when I remove the USB-C cable, use the computer, and then reinsert it into the system.

Oh, before someone asks, I've tried all three of the USB-C connectors on my MacBook Pro. I've tried it without the Dock, and I have also tried it with other monitors I have connected to my other Mac in my home office. This issue occurs only on my MacBook Pro and only when I switch from clamshell (or open) mode to using only the laptop screen, then back to the external monitors. (And as I said, my bandaid fix is to reboot, but what a pain in the you know where)

Now that I can reproduce it, I'm writing up a bug report for Apple. Good luck, everyone!

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.

The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

<< I don't understand the logic behind recommending that the cable is the issue. The cable worked moments before I did the macOS 26 upgrade, with the same computer, monitor, and cable. Once the software was updated, the setup didn't work. This thread has many views compared to all other Tahoe topics, likely meaning others are searching for this same issue. >>

This is not an issue of BROKEN cables. Users are finding their cables do not meet the demanding requirement of connecting high-end displays to recent Macs with the latest MacOS.

Modern displays are running very close the the maximum speeds the logic can switch, and still be recognizable at the other end. And MacOS has become increasingly demanding that NO transmit errors occur, such as those caused by low spec cables or cables that exceed the maximum length.

<< The USB-C to USB-C cable I was using came with a monitor. I also tried a USB-C cable that comes with other Apple products. Both have the same result. >>

Cables "shipped in the box" are infamous for being 'lowest bidder' cables, just good enough to keep you from returning the device the same day you bought it.

Found cables of any description might be anything.

That is why I spelled out what specifications high-end cables for this use must have. Cable LENGTH is not negligible.

<< I will try to order a Thunderbolt cable. If that solves the issue, I will report back. However, it's silly that a software update now requires purchasing a different type of cable. >>

ThunderBolt cables should meet those specs as well, but are not a requirement. Be sure to limit length to ONE meter or less for USB-C displays.

For genuine ThunderBolt displays, their transmit speeds are TWICE as fast, and their length limit is 0.5 meter, maximum.

The Thunderbolt cable did not fix my external monitor issues. Spend a good amount of time with Apple support, going all the way to reinstalling the OS.

Diving deeper into my monitor, there are reports online that my monitor model can have issues with the USB-C port on the back. It's very odd the timing to line up perfectly with the Tahoe update, but the USB-C/Thunderbolt driven monitor will not work now. If I use an HDMI cable to Thunderbolt, the monitor acts normally, but the cable does not supply power.

So, for a solution, I will need to plug in 2 cables to the MacBook and travel to and from work with a power cable. Not a big issue, just not as expected when it worked fine before Tahoe.

<< Now only one monitor will connect unless I switch the other one off and on again. >>

Powering off and on displays may not be the best way to troubleshoot, because of the way MacOS detects displays.

The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Adding my solution for the algorithm: Using a proper TB cable.

I was using a random USBC cable to connect my M4 MacBook Pro to a TB4 dock with two monitors. This worked fine before macOS 26, and currently still works with my Linux laptop.

Anyway, this morning my MacBook couldn't see more than one monitor at a time connected to the dock.

I switched to a proper TB4 cable between my MacBook and the dock and both of my screens were immediately detected.

Good Luck!

So, I actually finally figured out how to fix my issue tonight, and I'm almost embarrassed that it took me this long to think of it. Wavlink support tried their best, but I managed to figure this one out first:

Plugging in my MagSafe charger appears to completely break high speed thunderbolt connectivity. It will work for smaller things like mice and keyboard, but any kind of hub or display will just fail to establish a connection while I'm charging with the MagSafe port. Once I unplug the charger, the dock works great, and so does plugging my displays directly into the laptop.

I have no idea why this would be working fine and then break after an update. But my particular use case is fixed so I'm not going to worry about it too much at this point. So if your monitors and/or dock are busted after a recent update, just try unplugging the charger and see if that helps. If you're just using a passive device that won't charge the laptop though, that might still be a dealbreaker for you.

Can’t believe it, but swapping the cable actually fixed it.

I had the exact same issue — after the OS26 update, my monitor just stopped working with the original type-C cable. Wouldn’t even charge the MacBook through it. Basically like the cable wasn’t plugged in at all.

Grabbed some random cable from a dead Thunderbolt hub I had lying around, and boom — the monitor came back to life!

I have the same issue with my docks. I have a WD19TB and a WD22TB4 dock, and I have the same issue on both of them. Plugging into the dock previously would give me access to two monitors, usb ports etc. (on M3 Max 14"). But now, the "first time" I plug in my dock into a specific port, it will work. But unplugging it and plugging it back in will result in no connections at all. (or trying on a new dock)

Additional symptoms are that the system info / thunderbolt section takes FOREVER to refresh.

Demonstration here: (before plugging into my one remaining "untested" port)

Result: Dock works (both monitors +USB KVM), but USB 4 text disappears from system info, Port reports nothing about device connection or anything (future connections will not work)

Restarting my computer restores all three ports to their original state (ie: I can connect each port one time again)

This is on both Tahoe MacOS 26 and 26.0.1

Just to chime in with the same / similar problem.

I have my Mac Mini M4 hooked up to a RayCue M4 hub. Since upgrading to Tahoe the second display stopped working.

For different reasons I recently purchased the UGreen M4 dock with DisplayPort connector to replace the RayCue. Unfortunately that also doesn't send a video signal to my second display.

From work I have a cheap USB-C hub with HDMI-out. That doesn't send a signal to my second display.

But luckily I also have a cheap USB-C to HDMI / VGA / DVI dongle.

Guess what... that works, as long as it is connected directly to the M4. Moving it to another USB-port from one of the docks, there's no video-output.

To my guess video-output is for whatever reason disconnected when there a hub in between.

Oddly, this is the post that helped me fix my display issue. Huge thanks!

Post-Tahoe install, my 2020 iMac suddenly stopped connecting to the 2nd monitor. I have always used the appropriate 1M fast lighting cord in the correct ports

None of the following worked: restarting, sleep to wake mode, unplugging and plugging each end of the USB cord, or holding down the option key in display settings to "detect displays"

What did work:

1. confirming my iMac was not reading anything from that USB port
2. un-plugging and plugging didn't work previously
3. so this time flipped the cord so each end went into opposite ports (using the same iMac port and an alternate lighting port on the secondary monitor)
4. iMac monitor went immediately black, and the secondary monitor appeared to turn on, but was also black for ~10 sec
5. Now everything is working perfectly!

Not a hardware engineer, but it warrants investigating whether the update is pushing too much electricity to the ports of some monitors and/or cords

Noticed a recurring pattern in these threads of people simply replacing their cord to fix the issue. So if your 2nd monitor has multiple lightning ports, recommend flipping + utilizing an alternate lighting port on 2nd monitor before buying a new cord

The reason I think the flipping made a difference is that during my initial unplugging/plugging testing, I tried the original end in the 2nd lighting port on the other monitor to no avail

<< 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:

Product Feedback - Apple

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.