When I go to a site, I have the parts to fix a certain, specific problem.
Usually, the equipment company--Dell, IBM, Lenovo, Sony, Apple, or HP--and the customer have taken steps to determine what the problem is. I simply fix what I'm told to. I have a little leeway if, for example, the symptoms don't match up with the problem or I've got the wrong parts; however, many of my fixes are hardware swapouts.
On the rare occasion that I need to extra diagnostics, I have a few tools at my disposal. In the long run, however, I'm expected to go in, get the job done, and get out...and SMILE because I represent Dell (or Sony or whoever).
I'm trying to avoid the soapbox, but like any other job this has its ups and downs. Just gotta roll with the punches.
I've got a call list of eight tickets for tomorrow. Another day at the zoo.