It's a USB loopback connector.
Wet finger. No but srsly try a usb that you know workd like a mouse or keybored in that slot. ^^
A Loop-back plug.
Loop-Back Plug
A loop back plug.
To test an Ethernet port, you can use a known working Ethernet cable and connect it between the port on your device and a functioning network device, such as a router or switch. Check for a solid green or orange light on the Ethernet port, which indicates a successful connection. You can also use network testing tools or software to further diagnose the port's functionality.
If you suspect the sending unit may be faulty just replace it with a new one, they are not expensive but a little difficult to remove and replace.
There is no test port.
If the battery, drive belt and wiring are okay then I would suspect the alternator is faulty. Your local auto parts store can test the charging system for you at no cost.
The USB port on your lamp may not be working due to a faulty connection, a damaged port, or a power supply issue. It is recommended to check the port for any physical damage, try using a different USB cable or device to test the port, and ensure that the lamp is receiving power properly. If the issue persists, it may require professional repair or replacement.
The Villager does not have a test port. You need to tee into the line at the fuel filter.
Some tools are mircoscope,test tubes, test strips.
If the test switch is faulty then there is no convenient way to determine if the GFCI is functioning, and technically, if the test switch is faulty, then as it is a part of the GFCI, the GFCI is faulty and should be replaced.