this!
I tell our people not to take offense to the customer not wanting to pay for the diag charge. I tell them to inform the customer and they will understand it, if they don't understand it we liken vehicle diagnostics to medical diagnostics, it is NO different in the meaning. We use, books, diagnostic equipment, self knowledge, learned knowledge etc in order to form a "diagnosis" and the same in the medical field, ALL of that costs money to make available to find out what is wrong with the car.
Knowledge is power, use it to your advantage and you will gain a customer rather than insulting one, take the time to educate them, it will pay off for you.
We have also had customer turn and walk out, but those are the customers that didnt have the money to repair the car anyway, that is a fact in 95% that walk out. The other 5% was we didnt explain it enough or well enough for them to understand or be reeducated.