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Yesterday everything was working fine. I had my MacBook Pro (four USB-C ports & touch bar) connected to my LG UltraFine 5K monitor (official model name: LG 27MD5KA‑B). At the end of my day's work I turned everything off. In the morning I turned everything back on but nothing seemed to work.

I proceeded to disconnect the monitor from the MBP and there the latter turned on without problem. I tried hot-plugging back the monitor and the MBP's screen turned black. Event the touch bar was unresponsive. So I tried all sorts of combinations in order to solve the problem:

  • Turn off the MBP, and unplug everything from the mains, wait a moment and reconnect everything;
  • MBP PRAM reset;
  • MBP SMC reset;
  • Removed all connected peripherals (either directly to the MBP and/or to the monitor);
  • Tried all the USB-C ports of the MBP;
  • Tried with another touch bar MBP;

One thing that half works is when the LG UltraFine 5K is connected to the mains and I hook an MBP with the lid close. In this case the monitor lits up, although very dimly, I can see the Apple logo and then suddenly the logo is spread horizontally across the screen and lower quarter of the screen is garbled (everything very faint). If I open the lid, only the caps lock seems to work (hinting that the computer has not crashed). If I unhook the MPB it responds without problems after a few seconds. Reconnecting it and the MBP screen turns black and the computer becomes unresponsive (apart from caps lock). I can, however, access the computer through remote login (SSH) and managed to shut it down or restart it from there (sudo shutdown -h now or sudo shutdown -r now).

I even downloaded the LG Screen Manager, but the app seems totally useless in my case (event the copyright notice seems strange showing "Copyright 2016-2026 LG Electronics Inc.").

Any suggestions on what to do next (apart from bringing the apparatus for repair)?

System used:

  • MBP (13", 2017, 4 thunderbolt-3 ports, Intel Core i5 @ 3.1 GHz, 8GB RAM, Intel Iris Plus Graphics 650 1536 MB);
  • MBP (13", 2017, 4 thunderbolt-3 ports, Intel Core i7 @ 3.5 GHz, 16GB RAM, Intel Iris Plus Graphics 650 1536 MB);
  • Both MBP are installed with macOS Mojave Version 10.14.2
  • LG UltraFine 5K (aka 27MD5KA‑B);
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    Try these things first and report back: (1) Plug the monitor into a port on the other side of the MBP, (2) Remove all other connected units on the MBP and monitor - i.e. USB devices, etc., (3) Try with a different cable - note it must be a Thunderbolt 3 cable capable of supporting 5 amps.
    – jksoegaard
    Commented Jan 10, 2019 at 9:44
  • @jksoegaard. Thank you very much for your comment. As a matter of fact I tried all your suggestions but the cable. Will buy one ASAP and try it out. (Will also update my post in order to reflect the things I did but forgot to mention.)
    – pdeli
    Commented Jan 10, 2019 at 16:23
  • Everything here, especially the part where you tried another MBP, points to a failed LG monitor. It's not unheard of. It's time to call LG support on this one.
    – Allan
    Commented Jan 10, 2019 at 16:29
  • @Allan. Yes you are right. The only thing I will try before doing that is what jksoegaard suggested: another USB-C to USB-C >5A cable…
    – pdeli
    Commented Jan 10, 2019 at 16:34
  • You mean a USB-IF certified cable. But why? Are you trying to charge your MBP off the LG? If so, is it charging? The symptom of the image being "dim" is consistent with a back light issue, not a cable.
    – Allan
    Commented Jan 10, 2019 at 17:10

1 Answer 1

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It sounds weird but anothet thing to try is if you have the Macbook touching the LG5K display in any way it causes some sort of earthing issue. As you know the Macbook aluminium case is a conductor and there are minute amounts of current generated on the shell by the Macbook Pro itself. If these currents find their way to the LG5K display it causes erratic behaviour (display turns off for a second or two) or wavy lines.

Also pls be aware not all Type-C's are the same - a Type C1 (TH3 mode) passes a 5K display signal whereas a Type C2 (USB3.1) does not.

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