Wet finger.
No but srsly try a usb that you know workd like a mouse or keybored in that slot. ^^
It's a USB loopback connector.
A Loop-back plug.
Loop-Back Plug
A loop back plug.
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.
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.
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.
There is no test port.
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.
-- Using appropriate test equipment and logical test procedures, identify the component, section, or connection that has failed. -- Using suitable mechanical and electronic tools and materials as required, repair or replace the faulty connection, component, or section of the radio.
Strict scrutiny test