I often tell customers that I like watching "House"

Fixing cars is a bit like medicine (Particularly modern ones), Just because you are an expert doesnt mean that you wont go down a couple of dead ends before getting to the right awnser.

Indeed, sometimes it is an essential part of the process. (Think about the number of times you have spent considerable time "Checking" say camshaft timing because although you dont really think it is the problem you nevertheless feel that you have to make sure it isnt?)