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I run a Late 2012 Mac Mini running 10.13 High Sierra with a Dell QHD monitor via DisplayPort. I've seen two different cases where as far as I can tell the Mac is running (HDD light flashing, etc) but the monitor shows as idle:

  1. When it goes to sleep from lack of use, the monitor sometimes won't wake up (like once every few weeks, I leave it always-on). I already run caffeinate -s in a console window and I think the Mac itself is running, but sometimes the monitor thinks otherwise and I have to reboot.
  2. Today, I turned the Mac on for the first time since turning it off over Christmas - first cold start for a while. The monitor didn't come on, just sat in standby mode (not no-signal). I pulled the DP cable and plugged it back in and now it came to life, which doesn't work in case 1 above.

These things make for a really annoying start to the day and I can never find what might be wrong. I already chased some ideas - I thought the Mac was going into deep sleep and not waking, but now I am fairly convinced the Mac is running but not waking the monitor and I don't know what to try next.

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Assuming your cables are good and connected firmly, I would try resetting the NVRAM and SMC (in that order).

Resetting NVRAM

Your Mac use Non-Volatile Random-Access Memory (NVRAM) to store a whole range of settings. Here’s how to reset this:

  1. Fully shut down your Mac mini
  2. Press the power button and then press the commandoptionPR keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
  3. Hold those keys down until your Mac reboots again and you here the startup chime for the second time
  4. Let go of the keys and let your Mac mini reboot normally

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Resetting SMC on a Mac mini

Follow these steps for resetting the SMC on your Mac Pro:

  1. Fully Shut Down your Mac min
  2. Unplug the AC power cord
  3. Wait at least 15 seconds
  4. Plug the AC power cord back in
  5. Wait at least 5 seconds
  6. Switch on your Mac mini

Now use your Mac mini as usual to determine if the problem still occurs.

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