External display not working after installing MacOs Catalina

Hmmm....Strangely enough, it seems that the USBC port you plug the monitors into may also be causing the issue.

When I had it plugged in to the port furthest away there was no monitor detection.

However, when I plugged them into the port closest to me, it seemed to detect the displays straight away.

Perhaps, try changing the ports around to solve the issue.

APPLE PLEASE FIX.

I have iMac 27" 2017 that worked fine with an external monitor ViewSonic VA2759-SMH plugged in directly from HMDI to USB-C this morning before upgrade to MacOS Catalina. Now the monitor does not show up in displays in system preferences even after "detect displays". Thoughts or wait for patch?

I have similar problem on my 2018 Mini and this worked for me; under "Energy Saver" in settings make sure "Prevent computer from sleeping automatically when display is off" is CHECKED.

You also may need to reset the SMC

Here are the instructions for resetting SMC. They do not require a working monitor.

https://support.apple.com/en-us/HT201295

I understand that not having your monitor working is serious but we are users like you, trying to help so shouting is not welcome (writing in all caps is considered shouting).

Maybe this might point you in the right direction.

I fixed my full time pink screen issue, here is the solution. I think it might apply to your issue as well. For those of you without working screens, I suggest finding a screen that does work temporarily, and installing Teamviewer, or some other free program to remotely log into your machine. After doing so, connect the nonworking screen, start the computer, and log in remotely, then follow the steps 1-5 below. you will have to switch back to the working screen for the steps 6-15

The reason it is having an issue, is after the update, the signal being sent is YCbCr instead of RGB like your monitors are expecting. Some people with panels like mine will get pink, others likely get no signal recognized. This script works by setting up a profile for your specific monitor, and having it be forced to be RGB, if you connect it to another model monitor with the same issue, this process will need to be repeated for that monitor as well.

1. Download this script: https://gist.github.com/adaugherity/7435890
2. Place the patch-edid.rb file into the /Users/ directory
3. Run this command in Terminal: ruby /Users/patch-edid.rb
4. Take note of the DisplayProductID and DisplayVendorID file names. These will vary based on your monitor.
5. Move the output file named "DisplayProductID-XXXX" from /Users/~username/ to the /Users/ directory
6. Reboot your Mac in Recovery Mode by pressing Command+R as soon as you see the Apple logo after rebooting
7. Open Disk Utility in Recover Mode
8. Check to see if "Macintosh HD" is mounted. If it isn't you will need to mount it before proceeding.
9. Exit Disk Utility
10. Open Terminal in Recovery Mode
11. Change to the Overrides directory via "cd /Volumes/Macintosh\ HD/System/Library/Displays/Contents/Resources/Overrides/"
12. Create the folder with "mkdir DisplayVendorID-XXXX".
13. Run this command in Terminal (make sure to include spaces):
14. "cp /Volumes/Macintosh\ HD/Users/DisplayProductID-XXXX /Volumes/Macintosh\ HD/System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-XXXX/"
15. Restart computer and the monitor should now be in RGB mode.
16. Check to make sure it has taken effect by going to your "system preferences" and clicking "displays". If your display name at the top bar does not include "forced RGB mode (EDID override)", it did not take effect.

Hope this helps everyone! if you have issues look in the comments on the script page, the author is active.

My machine is a Mac mini (2018) i7 64GB 1TB. My monitor is a 28 inch Hannspree HF289H monitor

I have a very similar setup, Late 2012 Mac Mini using a d2 Thunderbolt HD to daisy chain both a 27" Thunderbolt Display and 2012 27" LED Cinema Display and everything worked perfectly fine on High Sierra before I decided to update to Catalina. Now the problems I have are absolutely ridiculous:

1. If I disconnect the HD, my Thunderbolt display goes completely black and I have to force Shut Down my Mac Mini
2. If I restart, my LED Cinema Display doesn't turn on
3. Apps like TeamViewer no longer work which is extremely frustrating because we use it extensively for work

What does work:

1. Putting computer to sleep and waking still works fine
2. Starting screensaver and returning still works fine

Apple should've never allowed this update for 2012 Macs, it totally wreaked havoc on my machine and there is no recovering from it. I would've gone back to a previous backup but but the time I figured out that nothing was going to fix it a week had gone by and rolling back would've deleted a ton of work I had completed for work.

Not happy. Booooooooo Apple.

I had the same issue with my external monitors which were connected via HDMI.

They appeared black, despite the fact that they were visible in 'Arrangement' window, and I could see the mouse moving.

Fixed the issue by clicking on the 'Mirror Displays'.

All screens turned on, and then I turned mirroring back off to use as per usual.

This solution did NOT work for me.

Bought a Startech.com USB-C Adapter from Amazon namned: "USB-C to Mini DisplayPort Adapter - 4K 60Hz" and it did not change anything. Same result as before with the Apples USB-C adapter. Problem must be Catalina based and not hardware. Writing this so others can save their money and not try this or similar adapters to solve this problem.

For reference I use a MBP 13" (2018) + Apples Cinema display with a display port as external monitor, this worked just fine before installing MacOS Catalina...

This happed to me, too.

Unplug the external monitors and restart your Mac. Once it's up and running, plug in the first external monitor, wait for it to light up, then plug in the 2nd one.

Worked for me...

Update. I think the problem is with the HDMI in Catalina. I had an older monitor that didn't use HDMI, and it is working fine. On another thread, several individuals are having problems with HDMI plugged into their MacMinis and external monitors and Apple is supposedly looking into it.

I don't know if the cause and effect are there, but when I started this morning, the monitor was not working. My "Clean Your Mac" updated to Catalina and I ran it. Voila! After completion with no restart, my external monitor suddenly appeared. Since "Clean MY Mac" cleans the RAM, it might have been the problem. Thanks.

I don’t understand what you are saying. If you are talking about a program called “Cleanmymac” then you are using one with terrible reputation and that ought to be considered malware. In any event, a third party program does NOT upgrade your mac to Catalina or any other version of the OS.

This is the program I am using. https://macpaw.com/cleanmymac I've used for over a year with no issues. It is not the one that would pop up on less reputable sites a few years ago. CleanMyMac X did not update my OS system. CleanMy Mac X updated to handle Catalina. If you were confused by my sentence in my reply, I apologize. I've had no problems with malware. I also run McAfee AV and no malware ever detected. I appreciate you looking out for us Mac users.

It’s your mac, your choice.

Cleanmymac is known to delete files that should be kept alone and to cause many problems, which often require a clean install of macOS to solve.

The most seasoned users around here also recommend against installing third party antivirus like macaffee, but again it’s your mac.