I have a mid-2015, 15 inch MacBook Pro, connected to a Dell P2715Q with a Display Port to mini-DP cable. With the lid open, I get both displays working.

I used to be able to close the lid and it would switch to just using the external monitor, but I have changed something in the settings so that when it is closed, the display is black.

I am using SwitchResX, to change the resolution, and I think I changed the colour from Millions to Billions, which broke it. I now can't change it back as it is only broken when the lid is closed. I want to have just the single monitor. How can I change the settings back to normal so that it works properly?

The MacBook Pro is plugged in to mains power, and is connected a USB Keyboard and Bluetooth Mouse.

I use the exact same setup at work, with identical Keyboard and Mouse, but a different monitor, and that works fine. It appears that the configuration for the monitor in open clamshell mode is different to that in closed clamshell mode, and I'm trying to find out how to re-configure the closed clamshell mode when it does not display anything with it closed.

  • Just checking - is your MacBook plugged in while you're closing the lid? – Jackson1442 Dec 5 '17 at 4:07
  • Yes, it is plugged in. I will update the question – Xetius Dec 5 '17 at 16:09

To reset the external monitor settings:

  1. remove the Folder ~/Library/Preferences/ByHost/
  2. remove the SwitchResX Preferences
    • fr.madrau.switchres.*
    • fr.madrau.switchresx.*
  3. reset the NVRAM but holding down Cmd-Alt-P-R on startup.

After that, I was able to use the external monitor on its own.

I didn't actually remove the folders. I moved them to a different, temporary location, but they are not in the same location.

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Check your display and energy saver settings.

And then just make sure that you are following each step in this article so that we can properly troubleshoot the issue.


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i had the same problem but i wasn't using any resoliution programs like SwitchResX.

Removing the "ByHost" folder and than resetting NVRAM worked for me too. i am using an 16" macbook pro 6C base model with AOC 24v2q monitor. thanks for the solution.

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