Built-in Ethernet not working
Solution 1:
For a very short window (a few hours) - Apple pushed an update that disabled the ethernet driver for some of the Macs.
If you can connect to WiFi and run this command:
sudo softwareupdate --background
That would fix the issue if yours failed to operate due to this bug. The details are covered at
- https://support.apple.com/HT205956
Also some more details are on this post:
- Ethernet not working on MacBook pro retina 13