How can I troubleshoot or reset a Thunderbolt Ethernet adapter?

The solution is to try and reset as many network related system preferences as possible. To be more specific:

1. Unplug the Thunderbolt Ethernet adapter.
2. Create and Apply a new default network location (system preferences).
3. Remove all older pre-existing network locations.
4. Remove the following preference files (global and user specific found in /Library/Preferences /Library/Preferences/SystemConfiguration and ~/Library/Preferences):

com.apple.network*.plist NetworkInterfaces.plist

5. Restart your Mac.
6. Connect your Thunderbolt Ethernet adapter as well as the network cable.
7. Add a new Location inside the network system preferences (AGAIN!) and remove the one added previously in step (2).
8. Wait a few minutes (close the system preferences).

Here is what I observed while trying to solve my issue:

I always used to have two Thunderbolt related entries in my network settings:

1. Thunderbolt Ethernet
2. Thunderbolt Bridge

I believe the bridge may be added by default and the ethernet version may be added on demand, when you plug in an ethernet adapter.

While my connection was "broken", however, this Thunderbolt Ethernet service / interface never appeared or worked again, so possibly some of the settings files related to it were broken.

After removing all old references (from the old locations) to it as well as the listed preference files, I now get the following three default services listed, when adding a new location into the network preferences:

1. Thunderbolt Ethernet
2. Wi-Fi
3. Thunderbolt Bridge

If you don't get these three and you are using a Thunderbolt Ethernet adapter, you may have the same problem as I had and should follow the advice above.

There is a line in your Console Output that indicate the Problem your are facing:

28/02/2016 11:17:21.941 AM com.apple.kextd[55]: Failed to load /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleBCM5701Ethernet.kext - (libkern/common) general/unspecified error.

'Failed to load [...].kext' means that the 'Kernel EXTension' for your Ethernet Adapter is not loading, therefore your Thunderbolt Ethernet Adapter cannot work.

The Question of course is why it isn't loading. The Error Message "general/unspecified error" is not very helpful in this instance.

I would do the following:

1. Check at the location if the file is present (and has not a 0 size). It probably is, else the error would be 'file not found' or something similar
2. check permission, best with Apples Disk Repair Utility (found in '/Application/Utilities')
3. try to restore the file from a Time Machine Backup

Try these suggestions first and post your results please.

My Thunderbolt Ethernet connector never worked, i.e., it was neither automatically detected as I would expect, nor could I manually add the interface. This using the location Automatic. So from Network Preferences I clicked on Assist me…, then on Assistant, which opened /System/Library/CoreServices/Network Setup Assistant.app. This added another location and detected the connector and it worked (I then deleted Automatic but renamed the new one as Automatic). It's actually bridge0 with member en3. It's always up, even after disconnection, so you might want to set it as inactive.