External display not detected after macOS Tahoe update via HDMI

macOS has become progressively less tolerant of video data packet loss due to sub-par cables. There have been several reports of setups that worked in Sequoia not working in Tahoe, with nothing changed but macOS. 

Best recommendation at this point is to try a new cable. Be sure to high quality cables (HDMI should be labeled Ultra or Premium, USB-C cables should be rated for high-speed data transfer), and cables should be as short as possible (≤1 m, ideally...for Thunderbolt 5, ≤0.5 m). A random cable you found in a drawer might or might not work. 

✅ Solution

What finally fixed it for me was resetting WindowServer and related display configurations:

1. Open Terminal.
2. Run these commands one by one (you will need your admin password for the sudo ones):

# Remove global WindowServer preferences
sudo rm -f /Library/Preferences/com.apple.windowserver*.plist

# Remove WindowServer cache/database
sudo rm -rf /private/var/db/WindowServer

# Remove user-specific display profiles
rm -f ~/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist 2>/dev/null

# Remove ColorSync profiles linked to displays
sudo rm -rf ~/Library/ColorSync/Profiles/ByHost
sudo rm -rf /Library/ColorSync/Profiles/Displays 2>/dev/null

# Restart display services (this will log you out)
sudo killall -HUP WindowServer

<< bought different types of HDMI cable (female to male). >>

You should be using male-to-male HDMI cables of modest length. using male to female cables implies you are using additional devices with unusual connectors, which you did not name.

Best results are obtained from certified cables labelled ULTRA High Speed, and they are backward compatible for older versions of HDMI.

--------

For MacOS 12 Monterey and some later versions, some users got relief by deleting some preferences files, then Restarting:

Library/Preferences/com.apple.windowserver

-AND-

~/Library/Preferences/com.apple.windowserver.<UUID>.plist -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist

-- where [~] tilde is an active abbreviation for "current user"

-- and <UUID> is a string of hexadecimal numbers unique to your system, grouped as 8-4-4-4-8 hexadecimal digits

Restart REQUIRED after these deletions.

Totally agree. I've also upgraded to Tahoe, 26.0.1 bought different types of HDMI cable (female to male) - 4K and 8K, as well as a type c to hdmi coverter. It all didnt work in Tahoe 26.0.1 version, with the same exact setup, when boot in safe mode, Macbook Pro could pick up the external display again. Extremely frustrating trying to understand what's wrong.

kokularaj wrote:

I recently updated my MacBook Pro 14” (M2) to macOS Tahoe. Since the update, my external display is not being detected when connected via HDMI.

However, if I connect my iPad as an external display, both the iPad and the external monitor work. Once the iPad is disconnected, the external display also stops working.

https://discussions.apple.com/content/attachment/6b595d67-aeae-493f-a303-c283f4a315e4

[Edited by Moderator]

I would verify the cable connection it has been a known issue...

the macOS is increasingly fussy over sub-par cables for external Display connection…

high quality cables certified ULTRA HDMI cables or PREMIUM ≤ 1.0 m length, USB-C cables rated for high-speed data transfer

Also for best results the shorter the cable the better, ex . for Thunderbolt5 cables , ≤ 0.5 m length

ref:

Connect one or more external displays with your Mac

you can submit your Apple Feedback here: Product Feedback - Apple

Every time I update the system to a new stable release the HDMI port stops working properly. It can work for three days with one monitor and then it just stops. Usually it detects the external monitor but the signal is not going through. But... it works through an HDMI to usb-c adapter.

I bought the M1 Pro 14 for two things: the camera (all other macbooks had terrible camera back then) and the HDMI port. And after each release there are problems with both of them...

BTW, rolled back to Sequoia. Everything works like a dream again. Guess that will be my OS for next 10 years :D

kokularaj wrote:

Thanks for your reply. I intended to connect only one external display via HDMI, while my iPad is connected as a display over Wi-Fi. Interestingly, when I connect the iPad as a display, the HDMI-connected external monitor also works. This makes me believe it’s not a cable issue.

To be proactive you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple

Braunie-

If you do not have a recent local, disk-based backup, your computer is like a ticking Time bomb. You are only one disk failure, one mainboard failure, one crazy software, or one "oops" away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail. In addition, you never know when crazy software or Pilot Error throws away far more than you intended.

If you are using another direct-to-disk backup method that you prefer, and you currently have a recent disk-based backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast.  (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).

Attach your external drive and use

Settings > General > Time machine ...

... to turn on Time Machine and specify what drive to store your Backups on.  It may ask to initialize the new drive, and that is as expected. APFS format is default format if running MacOS 11 Big Sur or later.

Time machine works quietly and automatically in the background, without interrupting your regular work, and only saves the incremental changes (after the first full backup). Time machine backs up your machine — including every connected drive that is in a Mac compatible format. it can not back up Windows format drives.

Time Machine's "claim to fame" is that it is the backup that gets done. It does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there and be current.

How to use Time Machine to Backup or Restore your Mac:

Back up your Mac with Time Machine - Apple Support

it's the software. I am running an older thunderbolt 2 apple external, been using a TB2>USB C adapter (from apple) for years with no problem. as soon as I upgraded to Tahoe it stopped working. I tried a new adapter to be sure, nothing.

tyler.komar wrote:

it's the software. I am running an older thunderbolt 2 apple external, been using a TB2>USB C adapter (from apple) for years with no problem. as soon as I upgraded to Tahoe it stopped working. I tried a new adapter to be sure, nothing.

exactly what model older apple display?

exactly what adapter is this?

How long is the cable?

You're right. LG 40WP95C owner here, connected to M1 Studio Max. Sequoia 15.7.2 and Tahoe 26 changed the game when it comes to Thunderbolt connectivity. The LG cables that I've been using for the past years, connected to the same M1 Studio decided to quit, because Apple changed the software. So I went and bought the expensive TB5 Apple cable and a cheap Anker one. Both work now. So, you're ok with Anker or any that is 'certified' Thunderbolt cable.

We have 12× SD5000T5 EQ Thunderbolt™ 5 docks at the office, all paired with premium StarTech USB-C cables.

Everyone who updated to macOS Tahoe 26.1 (with M2 Pro) is having issues not a single dock works anymore.

Seriously, Apple? These docks cost nearly €500 each, not even counting the cables.

How do you mess this up so badly? And don’t even get me started on the memory usage in Tahoe 26.1.

I’m honestly at the point where I’m considering switching to Microsoft… these display issues are driving me insane.

what model 38-in LG display?

how long is that USB-C cable?