LG Ultrafine 27MD5KA-B provides power to laptop, but does not get detected by computer. No video.

Thread Starter


Joined Apr 18, 2020
Hi all! First time posting here. I'm a software developer that has worked with laptop repair in the past, but I am not seasoned in hardware. Seeking help on the following:

I have a LG Ultrafine 27" 5K 27MD5KA-B monitor that I have acquired. When I first hooked it up, my laptop would detect it and display video, but it would intermittently disconnect/reconnect every couple of minutes. It's only connection is a Thunderbolt 3 / USB-C connection, so at first I used the cable that was provided. My next step was to try a brand new Thunderbolt 3 cable directly from Apple. The display issues persisted until one time the video went out it it has never been able to be detected on the computer again.

I've tried all of the unplugging the power from the monitor, trying different ports on my Macbook Pro to connect to, resetting the PRAM/SMC on the macbook, switching thunderbolt 3 cables, etc, but to no avail.

My next step was opening up the monitor, since this monitor was out of warranty and I wanted to see if there was anything obvious inside that could be causing the issue. I've attached photos of the only two main circuit boards that are in the monitor. One appears to be the main board that has the ports attached to in and connects to the LCD cable. The other appears to be the power board. I couldn't notice any bulging capacitors or odd burns anywhere on the boards. There was a small button on the main board that was only accessible when the back case was removed. I pushed this button while the monitor was plugged in and it actually displayed a rotating image of color blocks to the screen (Red, Green, Blue, etc). This showed me that there was actually power to the display in some way. I'm assuming this is used at the factory to test the display for dead pixels and such.

One thing to note is that the USB-C port that connects the monitor to the computer is a bit loose. This seems to be a complaint for this model of monitor and people who have complained about the "wiggle-room" of the port usually seem to not have display issues. I'm not sure though.

Has anyone run into an issue like this or does anyone have a suggestion on what to try next?




Joined Sep 9, 2010
What about the MacBook? If it's failing to detect the external monitor, there's a chance the video card has a problem. I'm writing this on my 2011 MacBook that has lost its discrete graphics card and now can only use the Intel video. No external monitor anymore. (This was a common problem caused by AMD Radeon cards.) It'd be nice to rule that out before going too crazy on the monitor. And of course the reverse is true also, find a way to test the monitor with other sources, maybe using different input ports if possible.