External Monitor Not Detected After macOS 26 Upgrade

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

So I had the same issue and was able to resolve the issue today by simply buying a new thunderbolt 5 cable from Apple. I had a generic brand before that came with the monitor. For whatever reason, my mac book pro no longer liked that after the upgrade, but when I bought the new cable everything started working perfectly.

Same issue on my machine. Was working with a usb c cable, then after update to 26.1 it stopped working with my 40" lenovo screen. The only solution that worked after testing multiple, was to replace the usb c cable with a dedicated thunderbolt 4 cable.

M1 MacBook Pro (2021) connected to a hub, hub to the monitor with an HDMI cable. After installing OS 26, the external monitor did not come alive when I closed the lid on the MBP. Problem solved by rebooting the Mac and the hub, opening System Settings>Displays, and pressing the Option button to detect the external display. Had to select the external display as the "Main Display" while the lid was open. Moderately aggravating PITA; has happened before, I think, when updating the OS to a new version.

<< 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.

"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.

1. Go Here: Feedback - iPhone - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: on when this all began, how you troubleshot it, and how you came across this solution, thus far. Include the URL of this thread.
4. Proceed from there as necessary

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