External display doesn't work (shows nothing, black screen) if Macbook pro 15 (2018) is closed (clamshell).

Hello there Luckerman,

Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let's take a closer look as we do our best to provide some additional insight.

We understand your MacBook Pro while closed is not working the same way with your external display as it did before. To get started, do you use any other kinds of accessories with this Mac or connected to the LG display when this happens such as an external mouse or keyboard?

The reason we ask is it sounds like you've been using your Mac in closed mode which does have some prerequisites as mentioned here under the "Connect your display" section: Connect a display to your Mac Otherwise, it should be expected behavior for the external display to not come on if there isn't an external mouse and keyboard of some sort being used with the same MacBook Pro while it's closed.

We can also try resetting the SMC and NVRAM on your Mac before we test things again:

How to reset the SMC of your Mac

Reset NVRAM or PRAM on your Mac

Then test things out again and let us know how it goes!

We hope this helps get you pointed in the right direction.

Take care.

I am having the same issue since updating to 12.2.1

My HDMI port cannot output to anything aside from 720p or forced resolution into 1080p despite being connected to a 2K monitor that outputted without any issue beforehand.

My workaround solution is to utilize a USB-C to HDMI adapter and it works fine (outputs at correct resolution) but that is less than ideal as when my computer goes to sleep it sometimes flip-flops the external displays and I have to manually change back every time. Basically it doesn't remember which is left and which is right based on the cables/ports I use to connect to my computer (2021 14" MacBook Pro with M1 Pro chip)

Extremely frustrating.