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I hooked up my old 2012 retina MacBook Pro to an external 4k monitor via HDMI. It works fine while the lid is open, but as soon as I close it, various issues occur.

  1. Monitor won't work or receive signal if I unplug MacBook from the charger (monitor has its own power supply).

  2. Every now and then, monitor will shut/disconnect for a short while and come back online. I checked HDMI connection and everything seems to be fine (new monitor, new cable). What I did notice is that this flickering sometimes happens alongside charger light going off, which I think is related to point #1 above.

Does this sound like a bust charger that is going on and off for short periods of time, or something more serious with my MacBook?

Mac is set to never go to sleep be it on charge or battery, so I don't think this is an issue either.

This also seems to be unrelated to the option Slightly dim the display while on battery power. Just unchecked it, restarted and I am still getting the same issue.

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    This is called "Closed Display" or "Closed Clamshell" mode and it's required that your MacBook be plugged in to AC power.
    – Allan
    Sep 19, 2018 at 13:52

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Sounds like you already have the suspects identified (power supply being intermittent, faulty port, faulty dongle (if in use), faulty HDMI lead). If I were you, I'd start a process of elimination. Source another power supply (and/or extension cord, if in use), swap out the dongle/port/HDMI lead. Also, try a different refresh rate/resolution for the display and see if that makes as difference. I'm using an early 2015 MacBook Pro with an Acer UHD4K2K display via display port and the laptop closed. I've had problems with the display not waking at times.

Also, check your Magsafe port for debris. Could be dirty contacts making the charging intermittent.

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    This happened to me on a 10.15.6 MacBook Pro (16inch 2019). A few weeks ago I tried plugging in an old BenQ projector. It would project, then turn off repeatedly. I gave up. Normally I've been using an old Cinema Display 30inch for work. It's worked fine all year up until today. Now it's cycling on and off. I don't understand, all of the MacBook ports are fine. The dongles are new as of this year. Both displays are old, but it was only affecting the projector. Now both. Seems more software related than dirty hardware to me.
    – wsams
    Jul 18, 2020 at 20:46

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