Display will not sleep; screensaver stays on all night

Please launch the built-in Activity Monitor application and select the Energy tab. Select

View ▹ All Processes

from the menu bar, if it's not already selected. Also select

View ▹ Columns ▹ Preventing Sleep (or Power Assertion)

Click the heading of the Preventing Sleep column to sort the process table. Which processes, if any, show Yes in that column?

Could be a few things. You could try creaating a new user and see if the display turns off. Or start in Safe Mode and check.

OS X El Capitan: Start up in safe mode

You can also try resetting the SMC

Reset the System Management Controller (SMC) on your Mac - Apple Support

post back what worked or didn't.

It's best to unplug the AC cord from the back of the iMac when restting the SMC. Works better than from the AC Outlet for some reason.

Please remove "Plex Media Server" and test. If the problem is resolved and you still want to use the software, refer to its developer for support or an update.

Any third-party software that doesn't install from the App Store or by drag-and-drop into the Applications folder, and uninstall by drag-and-drop to the Trash, is a system modification.

Whenever you remove system modifications, they must be removed completely, and the only way to do that is to use the uninstallation tool, if any, provided by the developers, or to follow their instructions. If the software has been incompletely removed, you may have to re-download or even reinstall it in order to finish the job.

I never install system modifications myself, and except as stated in this comment, I don't know how to uninstall them. You'll have to do your own research to find that information.

Here are some general guidelines to get you started. Suppose you want to remove something called “BrickMyMac” (a hypothetical example.) First, consult the product's Help menu, if there is one, for instructions. Finding none there, look on the developer's website, say www.brickmymac.com. (That may not be the actual name of the site; if necessary, search the Web for the product name.) If you don’t find anything on the website or in your search, contact the developer. While you're waiting for a response, download BrickMyMac.dmg and open it. There may be an application in there such as “Uninstall BrickMyMac.” If not, open “BrickMyMac.pkg” and look for an Uninstall button. The uninstaller might also be accessed by clicking the Customize button, if there is one.

Back up all data before making any changes.

You will generally have to restart the computer in order to complete an uninstallation. Until you do that, there may be no effect, or unpredictable effects.

If you can’t remove software in any other way, you’ll have to erase and install OS X. Never install any third-party software unless you're sure you know how to uninstall it; otherwise you may create problems that are very hard to solve.

Trying to remove complex system modifications by hunting for files by name often will not work and may make the problem worse. The same goes for "utilities" such as "AppCleaner" and the like that purport to remove software.

This procedure is a diagnostic test. It makes no changes to your data.

Please triple-click anywhere in the line below on this page to select it:

pmset -g assertions | pbcopy

Copy the selected text to the Clipboard by pressing the key combination command-C.

Launch the built-in Terminal application in any one of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

Paste into the Terminal window by pressing the key combination command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.

Wait for a new line ending in a dollar sign ($) to appear below what you entered.

The output of the command will be automatically copied to the Clipboard. If the command produced no output, the Clipboard will be empty. Paste into a reply to this message.

The Terminal window doesn't show the output. Please don't copy anything from there.

Hi, if you want to turn off the display but not sleep. In the screen shot I posted above, slide the "Turn display off after:" to whatever amount of time you want before the display turns off and enable "Prevent computer from sleeping… "

As far as the screen saver, it will start at whatever time you have it set to. You can disable the screen saver if you want.

FWIW, the screen saver will "go black" once the display is turned off. Make sure "Prevent computer from sleeping…" is not enabled in the Energy Saver settings.