First, unless you have a recall or warranty issue there is no need to go to a dealer. You just need a good local mechanic (local = not a national chain place).
Second, is the check engine light on? If not, then there probably is no "diganostic" that will do anything other than make money for someone else. If the light IS on then you are somewhat unlucky to be in CA since I think it is the only state where you can't get auto parts stores to pull error codes for free. But ask at a parts store - what is the easiest & cheapest way to just get your error codes? If you have them and get them do not listen to what anyone says - just write down the code and post it. (It will look like P0123)
Anyway - the engine dies and then what? You're stuck and need a tow truck? Or you start right back up and go on your way? Or what? And if its anything but "starts right back up" please be as painfully descriptive as possible. No one here is there with you, and there is no one simple thing that could be going on. There is a long list and the more info the better.