How do you diagnose car problems when fault codes are missing?

One of the main reasons I built DIAGLO is how often real vehicle problems show no fault codes, or misleading ones.

I’m curious how others approach these situations:

– Do you rely mainly on symptoms?
– Experience / pattern recognition?
– Trial and error?

– Call "that" friend?

– Simple google search?

– Simple AI search?

I’d love to hear how engineers, mechanics, or car enthusiasts handle these cases.