The local Chrysler dealership here charges a diagnostic fee. the first time, i went in, they charged me $150 to pull the codes and told me to go back to the guy that changed the alternator, because it was a voltage issue - didn't look at anything else. They want to charge me $150 AGAIN, so do more diagnostics this time, since changing the alternator didn't ultimately fix the issue.
They're definitely making some money, by charging you to "investigate" over and over, but never actually fixing the problem. Same deal with my wheel bearing - I told them i was hearing grinding nose in the rear, $150 to diagnose the issue, and then $600 to replace the bearing on top of that. I don't find that to be very ethical practices, so I'm not going back to them.