Here’s the deal with dealership mechanics. Sorry for the lack of brevity in this post. Some of them are really good. But oftentimes you’ll run into the mechanic who has only worked on modern cars which have computers that spit out diagnostic codes, and has therefore not learned to actually diagnose much of anything himself even if he had incentive to do so, which he doesn’t.
This kind of mechanic will plug a code reader into your car, and then look up the code that gets spit out, and then look up a chart for what that code can mean and assume it does mean that, then charge you to replace the part rather than actually looking further to see if the part is, in fact bad. And if whatever’s busted isn’t something the computer monitors, he’ll be absolutely lost.
I’ll tell you two stories. Many years ago in college I had an old Honda with a bad fuel injector that made it stall randomly when driving. I didn’t know that because it was before I learned anything about working on cars. Took it in for repair to a mechanic who’d opened his own shop after leaving a dealership job. It got diagnosed as a bad distributor, replaced for several hundred bucks. Car still kept stalling. OK, it’s a bad ignitor, another several hundred. Nope, still stalling. Let’s try the coil. Nope. New spark plugs and wires! No dice. Gee, dunno what’s wrong with it. Take it to the dealership and no I’m not giving you any money back.
I did. Dealership saw that no engine codes were being sent (back when this car was made, their computers didn’t monitor nearly as much as they do now), and charged me $100 to tell me “Sorry, we don’t know what the hell is wrong with it.” So there I was as a college student a couple grand into a repair that never fixed the thing because my mechanics didn’t know how to troubleshoot.
Second story, much more recently. I’d been fixing cars for a good while by now. Just-purchased 2007 Acura TL. It shook when I’d brake. Took it back, and they machined the rotors, which is the usual fix, but it didn’t fix it this time. Took it back. They machined the rotors again. Still no fix. Took it back. “We’re gonna machine the rotors again.” "Um, no, you’re not, because it hasn’t worked twice and I don’t want rotors that are so thin you can see through them, thanks. Check the hub for excess runout. “Our book doesn’t say anything about that, but we’ll put new rotors on.” I let them do that knowing it wouldn’t fix it, but at least I’d not have brand new rotors that had already been shaved twice. That didn’t fix it, so they wanted to machine the bloody rotors again.
I offered the service manager a case of his favorite beer if he had the hubs checked for runout and they were within spec. He took me up on it, and look! One of the hubs had excessive runout which makes things wobble and caused brake shaking. Replaced the defective hub and it’s been fine for over 100,000 miles. Fortunately all of this was warranty work or I would have been out a lot of money again fooling with their bad decisions.
Moral of both stories: It’s not a 100% rule, but dealership mechanics can be very, very bad at diagnosing things because many of them will rely too heavily on what a computer or a manual is telling them and not sit down and think about the problem for a few minutes. They aren’t paid to think, they’re paid to replace parts, and if they replace the wrong part, the customer rarely wins the “give me my money back because you screwed up” fight, so it ends up not being a loss for the dealership and therefore there’s no incentive to become better diagnosticians. They can get away with it because they’re the dealership and everyone thinks the dealership must have the most expert mechanics available for their car!
Small local shops, on the other hand, their lifeblood is repeat business, and you’re not going to keep coming back to them if they charge you a boatload of money and still don’t get your car fixed. Eventually word will get around that they, well, suck, and they’ll go out of business, just like the guy in my first story did.
This doesn’t mean you won’t find a good dealership mechanic. There’s at least one, I think, right here on this forum, and if I could guarantee that he was the guy working on my car, I’d have it at the dealership a lot more than I do now. But it’s good to be aware that there are good and bad mechanics, and bad ones will cost you a boatload of money without fixing the problem, and will sometimes find problems that aren’t there and then charge you to fix those too.