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macrumors member
Original poster
Dec 31, 2013
I have an LG 27UD68 that used to run 3840x2160at 60Hz prior to High Sierra update. It now only displays at 30Hz and the resolution says 3840x2160 (low resolution) in system preferences. I can not change the refresh rate as it is greyed out at 30 Hertz NTSC. The monitor is connected Thunderbolt 2 to Display port out of my 2013 Mac Pro. Anyone else have this issue?
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macrumors regular
Jun 3, 2014
I have the exact same monitor and sometimes have this issue. When I reboot my machine it always goes back to 3840x2160 at 60hz. I am running the GM high sierra build. It’s seems to be a bit picky with the 4K resolution.
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macrumors newbie
Sep 26, 2017
Similar problem here; 2013 Mac Pro, dual D700s, the firmware update (MP61.0120.B00 & SMC 2.20f18) pushed with the High Sierra 10.13.0 upgrade looks to have altered how my two Dell P2715Q displays are now supported/operated upon boot and following boot.

Previously, with the left display (4K @ 60Hz) connected via mDP to Thunderbolt-1, Port-3, and the right display (4K @ 60Hz) connected via mDP to Thunderbolt-2, Port-2, all was good upon completion of booting to macOS, with the left display showing the login prompt (neither of the Dell P2715Q displays would correctly show option-boot/recovery menus, requiring the temporary use of an older HDMI 1920x1080 display).


Problems following installation of High Sierra 10.13.0:
  • The right display now showed the macOS login prompt (previously was the left display).
  • The right display booted to 4K @ 30Hz (and remained so after login)
  • The left display correctly remained 4K @ 60Hz at all times
  • The right display always showed weird graphics artifacting (blocky/multi-color/blinking/corrupted looking) when shutting down/rebooting the Mac Pro (I initially thought perhaps the D700 in Slot-2 was dying)

Failed attempts to correct this:

  • Power-down and resetting SMC and PRAM had no effect
  • Moving the right display from Thunderbolt-0, Port-6

Partial success to correct:

  • Power-cycling the right display resulted in the display being correctly driven at 4K @ 60Hz. However, after shutting down, the right display would need to be manually power-cycled every time following a power down.

Final success:
  • Assuming that the D700 in Slot-2 was ok (well, it was yesterday on 10.12.6!), steps were:
  • Set the left display to physically default as primary by moving the mDP cable to Thunderbolt-2, Port-2
  • Set the right display to be alternate by moving the mDP cable to Thunderbolt-0, Port-6
  • Remove the system and user display configuration plists (assumes damage or incorrect migration in the upgrade to 10.13.0), this allows macOS to recreate them with the corrected/desired configuration:
  • Close all windows/applications (including [System Preferences]),
  • Open [Terminal] to make the following command-line changes (use caution!):
sudo mv /Library/Preferences/ /Library/Preferences/
find ~/Library/Preferences/ByHost/ -type f -iname '*.plist' -exec mv '{}' '{}.BACKUP' \;
sync;sync;sudo shutdown -h now
  • Reset the Mac Pro's SMC by unplugging the power-cord, wait for 15 seconds, plug the power-cord back in.
  • Reset the Mac Pro's PRAM by locating the Command, Option, P, and R keys on the keyboard:
    • Turn on the Mac Pro,
    • Quickly, press and hold the Command, Option, P, and R keys until the Startup sound is heard a second time,
    • Release the keys.
  • Login to macOS upon boot-up,
  • Open [System Preferences], and configure the display(s) to reflect the desired scaling and positioning.
  • Close [System Preferences] (this will create the new replacement plist files)
  • Logout
  • Shutdown
After these steps, Mac Pro was correctly showing both displays in the correct default primary/alternate positions, with both displays at 4K @ 60Hz before and after login.

Hope that helps!

Of note, those original plists were significantly larger than those newly created plists were (perhaps "corrupted" over the years of swapping out different configurations of monitors, cables and Thunderbolt ports!):

 2.3K : /Library/Preferences/
15.2K : /Library/Preferences/

 4.5K : ~/Library/Preferences/ByHost/*.plist
18.6K : ~/Library/Preferences/ByHost/*.plist.BACKUP
Last edited:


macrumors regular
Jun 3, 2014
At first it would go back to 60hz after a reboot now i am most of the times stuck with the 30hz :( I have to fiddle a lot with the cables, turn it off, back on, rebooting, reconnecting the cables and then after 15 minutes i can get the 4k 60hz back... Very very annoying


macrumors newbie
Jan 2, 2020
And it's not working? What's the issue?
It wasn't working but I've fixed it now. I still don't know what the exact issue is, but I have found a workaround. If my monitor is stuck on 30hz, I have to follow these steps in this exact order:
- unplug laptop from Thunderbolt 3 cable
- restart laptop
- give some time for laptop to start, maybe unlock it
- plug the Tb3 cable in

If I plug the laptop it without restarting it, it seems to be locked at 30hz.
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