I lot of them dont want you to learn cause then they wont make as much $$$ off ya
it's like most service Jobs the more the customer learns the less income they have
also they dont want you askin well what was wrong with that part of they arent totally honest in their repalcement part area

like geting your front end alined on a car a lot of places will tell you that you need this part or that part that isnt nessarly needing replaced at that time
I'm not saying all do it but have heard of many doing it
and other services doin it as well

I once worked for a printing and copy repair company
they had a habit of replacing parts that in my option didnt need replaced
example one customer was told by my trainer that they needed a new belt
the very next call we went to turned out to be a broken belt
and the trainer had only put one belt in the truck that day
so he proceted to put the belt on (the one that he had told the other customer was bad) and adjust the machine
and then charged the customer for a belt ( was very unacceptable to me )
I didnt work there long needless to say

so if they say no I'd be watch full of their charges
unless they have proved to be trustworthy before this