Just over eighteen months ago, Apple pushed out a silent security update to its Incompatible Kernel Extension Configuration Data, which blocked loading of the kernel extension responsible for making the Ethernet port work on some models of Mac. I was one of those affected, and understandably unimpressed.

Until it happens to you, losing an Ethernet port in this way seems a trifling inconvenience: turn AirPort on, and carry on as if nothing had happened, until Apple comes up with a fix.

What you don’t realise is that, even though your Mac may not use it to connect to your network, that Ethernet port is crucial for your Mac to be recognised by the App Store, and for your Apple ID. With the Ethernet port gone – whether by software or hardware damage – almost every paid-for App Store app is disabled, and you’ll start getting bombarded with Apple ID warnings.

If you have tried to work out what is wrong, in the hope of discovering a way to reactivate your Ethernet port, but got nowhere, you’ll probably want to get your Mac authorised again, and able to access all those apps which you paid for.

The best approach is to remove Ethernet from the list of network connections in the Network pane. Select it in the list, and click on the – tool at the foot of the list. However, if you were just to restart your Mac now, it would probably still think it should have an Ethernet port somewhere, and your authorisation problem would remain.

To fix this ‘good and proper’, you then need to remove at least one preference file: /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist to be precise. While you’re there, it is often helpful to move /Library/Preferences/SystemConfiguration/preferences.plist too. Then restart.

You’ll have to set up your AirPort networking carefully again, in all probability, but your Mac should then forget that it ever had an Ethernet port. Its authorisation for the App Store (and elsewhere) will depend on the AirPort interface’s MAC address, rather than that of the missing Ethernet port.

It’s interesting to read Michael Tsai’s account of going through this same process after a thunderstorm caused electrical damage to his Ethernet port. At least in my case, Apple eventually rectified the problem for me, and my Mac didn’t need a new motherboard.

It also bears pointing out that, in the event of a sudden loss of the Ethernet port, one of your first actions should be to ensure that port is properly connected to a network, and to restart in hardware diagnostics or AHT, as detailed here. You’ll also need to be within range of an active WiFi network, or you may find that you get a code CNWxxx reporting a WiFi hardware issue, rather than another error (unspecified, possibly CNxxxx series) pointing at the Ethernet port.

If your Mac returns a code ADP000, indicating hardware health, the cause is most likely to be software. Keep a watch on your software installations, because what has happened once could always happen again. If you want to read exactly what I experienced, the summary is here.

In those days of El Capitan, we had one major diagnostic advantage: the logs, which were still old-fashioned and relatively uncluttered. I’m not sure how we’d cope now with Sierra’s unified log, in which any useful information would be buried in a torrent of confusing error messages.