Capt. Geoffrey Thorpe wrote:

Right. Now a mechanic just replaces parts until the fault codes go away...

I've not found that to be the case. Not in the last 15 years, at least.

It's nearly impossible to narrow things down to just one part, so even with
diagnostic codes to point the way, mechanics still tend to replace the easy
stuff first.
For example if you get a "oxygen sensor not switching" code, 9 times out of
10, the mechanic will replace the sensor. Then, only when that doesn't solve
the problem, will they will actually start to look for the cause of the
mixture problem (vacuum leak, etc.).

The one time that showed up on my vehicle, they checked for (and found) a vacuum
leak first thing.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.