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);
  • 1
    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 Jan 10 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 Jan 10 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 Jan 10 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 Jan 10 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 Jan 10 at 17:10

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