In addition to providing your auto repair shop with a diagnostic form, it's important not to jump to conclusions about what's causing the problem. You'll want to avoid giving the shop a list of potential problems you found on the internet or a list provided from a friend. It's true that your actual problem might be on that list - but which one? This places the auto shop in the awkward position of having to address everything on your list in order to avoid you saying "I told you so." This can lead to the shop spending more time on diagnosis -- exactly the opposite of what you were aiming for by completing a diagnostic sheet. That's why it's best to use diagnostic forms to clearly communicate the observed problem and to build a working partnership with your auto service provider.