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User profile for user: applemac345
applemac345 Author
User level: Level 1
8 points

External display not working after Catalina update

Hello,

I have a 2013 MacBook Air and use an LG external monitor connected via the Thunderbolt port. After the upgrade to Catalina v.10.15 the external monitor has stopped working. I've tried everything suggested online, disconnecting everything, restarting, checking for updates, checking Display preferences. When I first plug in the external monitor after a restart, both displays flicker non-stop, the Mac screen flickers between the desktop and the login screen. If disconnected, the Mac then ignores the external monitor and the monitor goes to sleep, without it ever being detected. Please can someone help?


Thank you

MacBook Air 13", macOS 10.15

Posted on Oct 28, 2019 3:48 PM

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User profile for user: Joubert
Joubert
User level: Level 1
9 points

Oct 31, 2019 7:56 PM in response to applemac345

After Catalina migration, the Samsung Ultra-Wide C34J791 monitor connected to a a 2018 MacBook Pro 13" i7 (via Thunderbolt3/USB-C) now briefly blacks out about 10 seconds after the MBPro establishes a signal connection to the external monitor, such as:


1. about 10 sec after "awakening" the external monitor from auto-sleep (from Mac's Energy Saver settings), or

2. about 10 sec after entering screenlock password when booting up with the external monitor connected, or

3. about 10 sec after connecting or powering-on the external monitor connected to the MBPro via T3/USB-C cable.

 

The brief (1.5 second) blackout usually occurs only once upon the signal connection and doesn't repeat until the external monitor auto-sleeps again and then is re-awakened. The problem also does not occur on the

Macbook Pro's built-in display - only the external monitor. Resetting SMC/PRAM doesn't help.

 

This worked perfectly in Mojave and seems to be a external display driver bug introduced by Catalina.


Any help/fix would be greatly appreciated! Thanks.


Equipment used:

2018 MBPro 13" i7 (in closed clamshell mode) (Intel Iris Plus Graphics 655 1536 MB)

2018 Samsung C34J791x external monitor (connected via Thunderbolt3/USB-C cable)

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External display not working after Catalina update

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