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Question: External display won't wake up from sleep

I have a 2017 iMac Retina 4k with an LG Ultrafine 21.5" external display connected using thunderbolt 3 cable (no adapters). Other folks in my office have this working just fine with 2017 macbooks.


The issue Im having is consistent and as follows:


The external LG display works except that it will not wake up from sleep when the iMac does. On top of this, the mouse and other UI components stutter when being moved. Unplugging and plugging back in the thunderbolt cable connecting the display fixes this and resets the displays and turns on the LG.


Its obviously frustrating to have to do this every time the mac needs to wake up from sleep or alternatively setting the mac to never sleep. I've tried various changes to the Energy Saver settings to no avail.

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Oct 17, 2017 7:46 AM in response to rkaufmann87 In response to rkaufmann87

I've made sure macos is up to date (Sierra 10.12.6), have restarted the computer many times and have tried every setting in Energy Saver.


Also may be worth clarifying that other members of my team, who are using the 2017 macbooks, are also using the exact same LG display I am without any problems. But I am the only one using the display with a desktop iMac.

Oct 17, 2017 7:46 AM

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Oct 17, 2017 8:41 AM in response to rkaufmann87 In response to rkaufmann87

Unfortunately this did not help. Although, the PRAM reset did not seem to work. Holding the instructed keys just repeatedly turned the backlight on and off and the computer would not restart to the loading bar until I released the keys. Tried this a few times.


I did do some other trouble shooting around the office. I used one of the macbooks here to make sure it wasnt a problem with the cable or monitor itself. I hooked up the macbook to this LG monitor and it seemed to work just fine and the display woke up from sleep and reboot.


The strange thing is that I then hooked up my iMac to a different external display, the exact same model, and it seems to work, which would lend me to believe it may not have to do with the computer but for some reason, this monitor and this computer combo is just no fun.

Oct 17, 2017 8:41 AM

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Oct 17, 2017 9:41 AM in response to darrenVpV In response to darrenVpV

darrenVpV wrote:


Unfortunately this did not help. Although, the PRAM reset did not seem to work. Holding the instructed keys just repeatedly turned the backlight on and off and the computer would not restart to the loading bar until I released the keys. Tried this a few times.


I did do some other trouble shooting around the office. I used one of the macbooks here to make sure it wasnt a problem with the cable or monitor itself. I hooked up the macbook to this LG monitor and it seemed to work just fine and the display woke up from sleep and reboot.


The strange thing is that I then hooked up my iMac to a different external display, the exact same model, and it seems to work, which would lend me to believe it may not have to do with the computer but for some reason, this monitor and this computer combo is just no fun.

Good work on your troubleshooting. Sometimes there can be cable connections (as you guessed earlier) that create problems, it will not hurt to experiment with different cables combinations and to see if one work and another does not. Experience has taught that not all cables and adapters work the same AND that the can fail too.

Oct 17, 2017 9:41 AM

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Nov 23, 2017 8:51 PM in response to darrenVpV In response to darrenVpV

I have the same issue with the 5K iMac and 27 inch LG Ultrafine. In another discussion someone wrote the following:

Just found the solution for this, the screen LG has is own power source and when you have it connected to the mac he is charging the mac through the USB-C, so if you have another cable (power adaptor) plugged in the screen won't be able to turn on after sleep because power priority will go to the power adapter, so the solution is simple if you have your macbook pro 2016 connected to the screen unplug the power cord because it will charge directly from the screen.


It seems from responses this fixed the issue for the MacBooks. Has anyone found a workaround or solution for the desktop IMac? It is frustrating having to unplug and replug in the cords. Thanks for any advice.

Nov 23, 2017 8:51 PM

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Question: External display won't wake up from sleep