In a previous life I was a licensed mechanic at a GM dealership.
i got the cars the "PartSwappers" couldn't fix.....
Scanners and diagnostics said which system was fucked up. NOT which part/parts.
Our shop also got cars from GM zone that couldn't be fixed at other dealers...
Chris.
Oh yeah, self-test or what is referred to as BIT (Built In Test) in the industry I work in is for the High School level educated techs. The engineers get the more difficult stuff. Having worked both aircraft level and back-shop avionics and automatic test equipment over the years the only thing BIT is good for is when doing a quick down and dirty self-test at power-up - nothing more. There's only so much self-test that can be engineered into a system for a myriad of reasons. When it fails to identify the problem the real troubleshooting starts.
The real fun one's are when a component fails BIT, pull it and send back to manufacturer and it comes back as serviceable without any repair being performed. Those are called "CND's" or Could Not Duplicate Failure Mode. Then we look into overall system integration among other things and may in some cases relax tolerances or change system timing.
You must know what you're doing to get the problem children.