This post is not going to cover the trite troubleshooting suggestions most sites give, and assumes the following:
- You’ve already been using your LG 4K monitor with your MacBook and it’s charged fine up until now, so it’s not an issue of “the monitor doesn’t supply enough PD power” because you can confirm that it does/did.
- It’s not an issue with your laptop’s USB port
For me, the problem began when I swapped the USB-C cable from my MacBook to my iPad Pro. When I unplugged the iPad, the charging problem began. After repeated unplugs and re-plugs the MacBook would send video to the monitor OK, and it could use items plugged into the monitor’s USB hub.
But the MacBook would only run on its own battery power and wouldn’t even acknowledge it was plugged into power. From this point, even plugging my iPad back in had the same issue.
The solution was fairly simple actually, but not something everyone would think to do right away.
I unplugged the monitor’s power cable itself, as well as the USB-C from the computer, and left it off for 10 seconds. Then plugged the power plug back into the monitor and waiting for it to turn fully back on. Then, plugged the MacBook back into it and everything was fine.
My guess is that something got confused internally within the monitor switching devices and stayed confused as long as the monitor had power from that point. Forcing it to fully shut down cleared it out and restored normal function.
Let me know if this helps you as well if you’re having similar issues!