I don't know of any mechanic/dealer who will charge gratis for a
misdiagnosis. The problem is that there are often a multitude of
possible reasons, and they'd lose money if they had to give the
parts/labor away. I have heard of maybe free diagnoses until they
got it right.
Guaranteeing the quality of the assembly and parts is usually
the most one can expect.
Exactly. Cars are pretty complex, especially these days. Sometimes a
problem can have several simultaneous causes, and fixing one cause might
make it go away for a while, only to have it come back. A stupid, basic
example is an oil leak - they can replace the obvious bad seal, but another
one might be leaking as well, just not as bad, and a few months down the
road, it's back again.
I had a car once that broke in the strangest ways. I wasn't too
car-inclined in those days, at least not as much as I am now, I couldn't
tell a timing belt from an accessory belt. Anyway, the car would break, the
mechanic would fix the obvious, and it would be something else. Several
times I had that car in to several garages, and in each one the mechanic
would tell me (with a geniune look of awe and astonishment) that he'd never
seen anything like it. I remember one occasion where the vehicle was
running with some critical piece broken. It wasn't running well though,
that's why I had it in, and the mechanic couldn't believe it would start and
run without that part. (Can't remember which part it was, like I said I
wasn't a car guy until I got my Subie and became interested.)
-Matt