Thunderbolt Ethernet connected AND not connected to Ethernet

I am settign up a new MacBook Pro (Retina, 15-inch, Mid 2015) / OS 10.10.3. I have connected a functioning ethernet cable (works without problem on my old MacPro) via a new Thunderbolt to Gigabit Ethernet Adapter. All seems to work ok. Internet works. System Preferences/Network Tells me that Thunderbolt Ethernet is currently active and has the IP address 192.168.1.131. Wi-Fi is off. Bluetooth PAN is Not Connected, Thunderbolt Bridge is not connected.

However, if I click on Assist me/Diagnostics - Network Status shows Ethernet and Network setting in red and ISP, Internet, and Server in green. When I choose network port configuration - Ethernet , the next screen in the wizard - Change your primary port configuration, has the Use Ethernet box unchecked. If I check this the next screen tells me: Not connected to Ethernet: This computer does not seem to be connected to a DSL or cable modem, or to an Ethernet network.

How is this possible? Is there some way to test whether the adapter is actually working or not... logically it has to be working, so why would Network Diagnostics tell me it is not.

I am about to go on a roadtrip where to a non wifi/ethernet only environment. and will be in dire trouble if it turns out the Adopter is in fact faulty. Any thought on how I can decipher these mixed messages would be much appreciated