I don't know that you're really missing anything because it's far as I know there is no such device at this moment. However soon after we record this tip I believe such a device will become available to the general public. There's a website automatic.com which is selling a gizmo that does exactly what you're talking about.
It hooks up to the computer in your car and it sort of lives in your car's engine and if your phone is nearby it connects with your phone and it updates information in their phone application to give you information on repair records and driving habits and when you see a little red check engine light your car you can give you more information about what that check engine light is complaining about.
It's not very expensive it's currently listed as $70 for presale. It's not quite out yet but it might be by the time you hear this from automatic.com, I ordered one for $70 at figures were trying to see what it does. I'll let you know later how it works out,