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Why is the second display flashing off and on?

879 Views 3 Replies Latest reply: Oct 11, 2013 1:22 PM by kabaker RSS
kabaker Calculating status...
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May 23, 2013 12:08 PM

Second display is a 24 inch Cinema LED display. I have tried restarting & zapping PRAM, but the flashing continues. After about 5 minutes it appears stable again but I don't know why. This has happened before, many months ago, and similarly stabilized until now. Thanks.

MacBook Pro, Mac OS X (10.7.5)
  • sterling r Community Specialists Community Specialists (3,240 points)
    Currently Being Moderated
    May 25, 2013 2:04 PM (in response to kabaker)

    Hello kabaker,


    Here is an article about troubleshooting external displays with your Macbook Pro named Apple computers: Troubleshooting issues with video on internal or external displays found here



    Check connections

    When using an external display be sure to check the following:

    1. If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
    2. Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
    3. Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
    4. If more than one video adapter is in use—or "daisy-chained"—troubleshoot by using only one adapter.
      • Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
    5. If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).

    Reset the system
    You can reset the Mac's parameter RAM and SMC.Reset the resolutionStart by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:

    1. Start up in Safe Mode.
    2. From the Apple () menu, choose System Preferences.
    3. Choose Displays from the View menu to open the preferences pane.
    4. Select any resolution and refresh rate that your display supports.
    5. Restart your computer.


    I know you said you have reset the PRAM so you can skip that part if you come to it.


    All the very best,



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