Sometimes the tester is bad
 

I have a $6 plugs into the cigarette lighter with LEDs charging system tester I bought from Radio Shack maybe 30 years ago. Its my tried and true first test for electrical issues, and its super simple, three LEDs, green, yellow, and red. No green with the car not running and battery is bad, yellow when running is good, and red is bad.

Plugged it into my wife's car when her phone wasn't getting charged and no green so I was sure the outlet was bad. Turns out a wire inside had broken (right at the point it was soldered instead of crimped) on the spring loaded tip.

Always test the tester.

Yep. For instance, last night I tested one multimeter's diode check with another multimeter.