MarkManKuch
Level 1 (0 points)

Q: External Monitor won't work after sleep

Recently, when I wake up the iMac from turning off the displays the external monitor isn't recognized.

I have an 27" iMac Retina with a 27" Samsung UHD external monitor.  My iMac is set not to go to sleep, but I turn off the displays after 20 minutes. when I wake up the computer the second monitor wakes up, the the desktop doesn't extend onto the second monitor. In Settings > Displays, the second monitor doesn't show up. Even if I power cycle the monitor or unplug/plug it into the thunderbolt port.  Reseting the iMac works.

 

This never happened until recently, I'm suspecting it may be related to the recent update to El Capitain, but I'm not 1005 sure.

 

Any suggestions?

iMac (Retina 5K, 27-inch, Late 2014), OS X El Capitan (10.11.2)

Posted on Dec 11, 2015 3:49 PM

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MarkManKuch

Q: External Monitor won't work after sleep

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  • by jwade5618,

    jwade5618 jwade5618 Dec 21, 2015 3:13 PM in response to MarkManKuch
    Level 1 (0 points)
    Dec 21, 2015 3:13 PM in response to MarkManKuch

    Mark:

    I'm no apple expert but I experienced the same problem as you, and I also concluded it had something to do with 10.11.2 .  I have a Mac-Pro purchased about 1 year before thunderbolt came out.  I have a 27" apple display and a 27" dell.  The Dell is my 2nd display.   After the Mac Pro goes to sleep and upon waking, the Dell monitor never wakes up and isn't detected by the Mac.  A quick work-around I've found to get the 2nd (Dell) monitor to be recognized again by the Mac is to do the following:

     

    use option+cmd+esc to close all programs but finder. 

    go to the apple menu and put the mac to sleep. 

    once the mac powers down completely, wake the mac up

     

    so far, this process has always caused the second monitor to be re-recognized, and is it quicker than rebooting the mac

     

    Alan

  • by MarkManKuch,

    MarkManKuch MarkManKuch Dec 23, 2015 6:06 PM in response to jwade5618
    Level 1 (0 points)
    Dec 23, 2015 6:06 PM in response to jwade5618

    Alan...

     

    Thanks for the workaround.  I tried it several times and it works sometimes. The monitor is recognized, but it doesn't get a signal so it goes back to sleep, or it works, or it doesn't work.  I try this a few times and when it does work, it's definitely faster than rebooting.

    Thanks

    Mark

  • by iM iMac,

    iM iMac iM iMac Aug 31, 2016 1:23 AM in response to MarkManKuch
    Level 1 (4 points)
    Aug 31, 2016 1:23 AM in response to MarkManKuch

    I had this problem with my Dell 2220L external monitor after upgrading to OS X El Capital 10.11.6. After contacting Apple Support they solved the problem by having me delete several corrupted preferences from the System Preferences and from the Library. Presto! I would never have known how to find these corrupted files without them.