My dad always told me never to take my vehicle to the dealer for repair. I did it one time anyway and they wanted $3000 to fix the camshaft, which they said was bad and needed replacing. I took it to a diesel mechanic in south Austin I trusted more and he said he had bad news, my camshaft sensor was bad and it would be about $300 to replace it. I told him what the dealership told me, and he asked how long they kept my truck. I told him I was only there for a little bit before they told me the problem. He then told me the only way to know if the actual part is bad is to tear into the truck and get to it and look at it, which would take hours, and there's no way they did that in the time they had it. He surmised they ran their little electronic diagnostic thingy and the sensor showed a faulty part. Then he told me it would be very rare for the camshaft to really need replacing, but my model truck was notorious for the camshaft sensor going bad, and he had checked the sensor itself and was confident it needed replacing no matter what was going on with the camshaft itself. He suggested I let him replace it and see what happens. So I did and it was fine. New sensor showed everything was A-ok. So I could've let the dealership replace a part that didn't need replacing, and I'd still have had a bad sensor, and I'd have spent $2700 more dollars than I actually wound up spending.
Short version--I never took a vehicle to a dealer again.