The skill floor is under the house: I have a 3-series BMW that was having odd engine hesitation but not throwing any codes. The dealership's mechanics could not fix it! They ended up throwing a ludicrous amount of parts at my car including some very expensive items such as a new transmission, new transfer case, new fuel injectors, plugs, coils, fuel pump, wiring harness. The bill, which was covered by the warranty, totaled over $10,000 and they still had not fixed the issue. They finally called in a guy from an Audi dealership and he fairly quickly diagnosed the problem as a faulty oxygen sensor. They had the car for nearly 60 days while I drove a brand new 4-series loaner car. So BMW did right by me, but holy crap I have never ever seen such lousy troubleshooting!