macOS Sequoia 15.1 Does Not Connect to Apple Thunderbolt monitor.

I have solved my issue. Thankyou tbirdvet!!!! I have a MacBook pro M3 and like others The thunderbolt display ceased to work after upgrading to Sequoia 15.2. As suggested, I disconnected the power for a couple of minutes and plugged it in and at the same time I disconnected the thunderbolt display’s built-in cable and replaced it with an old style thunderbolt cable. It’s working beautifully. Either the power recycle worked or I had a bad built-in cable from the thunderbolt monitor. Again thank you all and tbirdvet.

Sorted. Bypassed the built in TB display cable and now using the TB port on the monitor with TB cable and USB-C adapter in to my Mac.

My 2018 MacBook Pro 13 inch also immediately stopped detecting my older Thunderbolt Display after upgrading from Monterrey to Sequoia. I tried many things recommended but none worked. I got it working as follows: My computer has four Thunderbolt 3 ports. I switched the ports between my power input and Thunderbolt display (it uses a Thunderbolt 3 to Thunderbolt 2 adapter. The display came on immediately. Not sure why this worked, but I suspect the port I was using for my Thunderbolt display perhaps had some problem that did not affect it in Monterrey but did affect it in Sequoia. I had used that same port for many years, but now will use the new one. I tried swapping them back to the original ports, but then the Thunderbolt Display again did not work. When I switched back to the "new ports", I again could not get the Thunderbolt Display to work. Finally, it did work when I unplugged the power to the MacBook Pro. Anyway, it is working now. Try switching ports, and you might try unplugging the power to the MacBook briefly and see it that makes a difference. I suspect the power cord might have been a more important factor.

Hello community, I would like to give you a solution that I found by chance - it concerns monitors with separate cables - not permanently installed cables like Apple Studio monitors.

My installed cables were hot on the HDMI side, which is actually normal, but I wanted to make sure there was no defect there.

That's why I changed the cable - to a new 8K cable specifically for Thunderbolt4

All problems were solved immediately after plugging it in!

Research into the cables showed that my old cables were only compatible up to Thunderbolt3 and only max 4K

After the upgrade, my Apple Studio Max probably sends much higher data rates through the Thunderbolt4 port and some cables can't handle that.

Try these solutions, I would be happy to receive feedback.

Sorted. Bypassed the built in TB display cable and now using the TB port on the monitor with TB cable and USB-C adapter in to my Mac.

Got the same issue, I unplugged the monitor from the main power supply and reconnected it to the power supply after 1 minute. Works like a charm. Thanx for the tip @tbirdvet

As a long time member of this board (since 2000), and an authorized service tech (presently not employed by Apple) I hope all of you are aware that the only way Apple will be able to fix this issue, is if enough of you open a case number with AppleCare about this issue. It would also be a good idea for you to open a ticket with the monitor manufacturer stating they need to work with Apple's developer community to develop a firmware update to fix this issue.

In the past, the worst such peripheral issue I saw was with the release of Panther. It coincided with the Oxford 911 bridge for Firewire. Eventually hard drive manufacturers with the Oxford firmware were able to issue an update that would prevent hard drives from being bricked and people losing their data.

As a side note, I work in a place with HP laptops and Desktops. The laptops are able to use the camera built in to HP's curved display, but the desktops we have do not over the same USB-C cables. Suffice it to say there is clearly enough misunderstanding to go around about thunderbolt and USB-C standards, that such incompatibility while sporatic, is not surprising.

Since this issue appears to be happening to Apple Displays with Apple equipment the best everyone can do is call AppleCare, open a case number, and request Apple look into developing a firmware update to address these issues.

Make sure when you call AppleCare to get a case number. This ensures that they have a record of how extensive this issue is.

I have written steps on how to downgrade your OS to verify the equipment is not to blame, and something is out of sync with how Apple wrote the OS. It may be anticipating a firmware that was installed at some point in manufacture and not getting it. If a downgrade works, then you know the proof is in how the hardware communicates may need fixing:

How do I downgrade my Mac OS X operating … - Apple Community

Got the same issue, I unplugged the monitor from the main power supply and reconnected it to the power supply after 1 minute. Works like a charm. Thanx for the tip @tbirdvet!

Same here. Updated 2 different MacBook Air and one MacBook Pro to Sequoia and the Thunderbolt 27" monitor remains black. At the same time, using the same cable/adapter, a 3rd and a 4th MacBook Air that are not yet running Sequoia will connect perfectly fine. I tried disconnecting the display from mains power as suggested in this topic, to no avail. I do not have a TB cable to connect to the port in the back.

Newer versions of macOS reportedly do not like to see display transmission errors, and will cut resolution or even cut signal entirely when they see said errors.

This is why running a Thunderbolt 1/2 cable from the TB2 side of the Apple TB3-to-2 adapter to the TB daisy-chaining port on the 27" Apple Thunderbolt Display is sometimes a workaround for the issue of "no video". Sometimes older Macs would display video despite the video connection being bad/flaky, while newer ones would not display video until a good connection was in place.

I'm not sure how that would explain having video going out when upgrading from, say Sonoma to Sequoia. I could be wrong, but I would have thought that Sonoma would already have been picky about "no transmission errors"

Unplug display from wall, wait 2 minutes and plug back in. This may reset the monitor.

My MBA M1 running 15.3.1 works perfectly with my TB 27" display. As already stated unplug the display from wall, wait 2 minutes and plug back in. This can rest the display.

Are you sure you are using the same cables and a configuration that worked before?

Could you possibly restore a backup to a separate partition to see if something hasn't failed in between the upgrades?

One thing I noticed in this thread at least some people are jumping from all USB-C power to Magsafe 3 power.

Is the issue still present when the computer is plugged into the Magsafe 3 power adapter designed to work with that model?