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Cannot control brightness of LG UltraFine 5K after wake

Menopause

Menopause

macrumors 6502a
Original poster
Feb 26, 2011
663
1,807
Hi guys,

I'm on a MacBook Pro 16" (2019) running macOS 10.15.4 Catalina. It's connected to an LG UltraFine 5K Monitor purchased directly from Apple in November 2019.

I've had the problem consistently over the past months that whenever my Mac goes to sleep and wakes up, I cannot control the brightness of the external monitor anymore: neither from keyboard, nor from the Touch Bar, nor manually under System Preferences → Displays → Brightness of the LG Monitor. The brightness stays stuck at the level it was at when the computer went to sleep. The brightness HUD shows the bars going left and right but the brightness does not change.

I can control the brightness of the MacBook's display.

The only way to resolve it is to unplug and replug the Thunderbolt 3 cable or toggle Mirroring / change resolution, that is, do something that would make the computer detect the display again.

No amount of SMC / PRAM resets, connecting the cable to other Thunderbolt 3 ports on the computer, trying a different cable, formatting the internal SSD and installing macOS from scratch, turning off Night Shift, turning off Auto Brightness, turning off Power Nap has been able to fix it. I've also tried a second display of the same model with the same results, which leads me to believe that this has something to do with Catalina. It's super frustrating.

Does anybody know if there's a solution to this problem? Many thanks.
 
Out if f curiosity, if you force the computer to go to sleep and then wake up, does the issue still happen? Also, curious if you boot to safe mode does the issue still happen?
 
So if I put my Mac to sleep right after it wakes up (and the brightness control stops working), and re-wake it, brightness control works again.
 
And the bug persists with a wipe and clean install of Catalina 10.15.5.

Would really appreciate any insight.
 
(i) I got two LG 24MD4KL-B monitors daisy chained to my Mac. I just recently got them from the company I work for and I have similar/same issues. At some point I can't control one or two of the two displays anymore.

(ii) Another more crucial bug: Data transfer stops from USB Devices attached to the back side of the Mac. If I for example have a wireless 2.4 receiver for my mouse plugged into the display, it will just stop working randomly until I restart.

(iii) It actually manages to crash my MacBook, if I go with different resolutions on different devices. Not every time but every 3rd or so. I had the LK 5K before and there it was happening every time I tried to connect a second QHD monitor after I connected the 5K. I was able to connect it the other way around. First the QHD then the 5k. That was most times ok for some reason. Im really disappointed by the software side of things on these monitors.
 
Last edited:
Menopause said:
Hi guys,

I'm on a MacBook Pro 16" (2019) running macOS 10.15.4 Catalina. It's connected to an LG UltraFine 5K Monitor purchased directly from Apple in November 2019.

I've had the problem consistently over the past months that whenever my Mac goes to sleep and wakes up, I cannot control the brightness of the external monitor anymore: neither from keyboard, nor from the Touch Bar, nor manually under System Preferences → Displays → Brightness of the LG Monitor. The brightness stays stuck at the level it was at when the computer went to sleep. The brightness HUD shows the bars going left and right but the brightness does not change.

I can control the brightness of the MacBook's display.

The only way to resolve it is to unplug and replug the Thunderbolt 3 cable or toggle Mirroring / change resolution, that is, do something that would make the computer detect the display again.

No amount of SMC / PRAM resets, connecting the cable to other Thunderbolt 3 ports on the computer, trying a different cable, formatting the internal SSD and installing macOS from scratch, turning off Night Shift, turning off Auto Brightness, turning off Power Nap has been able to fix it. I've also tried a second display of the same model with the same results, which leads me to believe that this has something to do with Catalina. It's super frustrating.

Does anybody know if there's a solution to this problem? Many thanks.
Click to expand...
I have this problem as well. I've run dual 5K monitors daisy-changed on a 15" MBP and usually one or both will stop responding to brightness adjustments after a while, even in the absence of putting the machine to sleep.

Same problem occurs with one of them linked to a 13" MBP. Have to unplug then replug it in.
 
Same problem here, except that it's a bit more persnickety and isn't fixed by unplugging/replugging, sleeping/unsleeping, or mirroring/unmirroring to redetect the display.

Rather, the only lame workaround that works for me is plugging it into a *different* Macbook, a Macbook Pro 15 inch 2019 (i9), running the same MacOS 10.15.7! Then I can adjust the brightness and switch back to the 16 -inch.

The problem does seem to be specific to the MBP 16-inch. I have other problems with external monitors on the same machine. After the Catalina OS upgrade, my Dell UP3214Q monitor stopped working with the MBP 16-inch through a displayport adapter, but it still works fine with the MBP 15-inch.

Why oh why did they leave out physical buttons on this monitor!? Simple is nice, but so is having recourse when something goes wrong.

- MacOS 10.15.7
- LG Ultrafine 27 inch
- Model MacBookPro16,1 (16 inch, 2019, i9)
 
Same problem here with a 15" 2015 MacBook Pro. I have not tried un-plugging/re-plugging in the monitors or changing the settings (which are both only workarounds). The other workaround which works for me is reboot. This is a nagging issue for sure.
 
Same problem, this bug only happens the first time I plug in the thunderbolt cable when my MacBook is in sleep mode. I guess you just have to wake up from sleep before plugging in the thunderbolt cable. One easy way to do this is with a bluetooth keyboard/mouse, just type a key or move/click the mouse before plugging the thunderbolt cable in and it will wake up the MacBook (this works even if the MacBook lid is closed). Otherwise, unplugging and replugging again (which wakes up the MacBook in the process) also works. I've submitted a bug report to Apple, hopefully they'll see it.
 
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This is an old post but for what it's worth the solution that worked for me is to download LG Screen Manager app, open it and go to Settings and Factory reset. The display will go briefly blank and then full brightness controls can be used
 
I use BetterDisplay on all monitors, even the ones supported by macOS. There is a free and a Pro version. The Pro version adds a large list of features too long to list here.

I started to use it with my Dell monitor, which is no longer supported by Apple Silicon starting with Monterey. It has been very reliable and adds extra features.
 
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