Flashes and pulses.
I was staring at an archaic diagnostic system on a 1992 Volvo 940 wagon. It was located underneath the hood, inside a plastic cover, with six little holes for each one of the six digits, along with a cheap plastic wand.
What came out was morse code. Three little reds, stop. One little red, stop. Two little reds, stop. Code 312. Time to visit the brickboard, where the code could be translated to about fifteen different potential issues.
21 model years later, and we’re still not quite there yet.