It is often perplexing why customers would ask this question. Being able to discern the reason for a particular problem with a car is arguably the most important ability a technician possesses. To do this accurately, he (or she) must rely upon years of experience, deductive reasoning, the correct diagnostic software, and intuition. The diagnostic equipment alone costs thousands of dollars rather than the $100 scanners used by most automotive chains. The technician must be able to retrieve the fault codes, interpret them, and run the appropriate diagnostic tests in order to confirm the validity of the fault. We have seen numerous attempts by mechanics to throw parts and labor at a particular problem in an effort to quickly solve it, but to no avail. I guess these mechanic diagnostic charges were free...and after all, you get what you pay for.
I am amused by people who think computers fix cars and expect that simply hooking it up to the vehicle will indicate exactly what is wrong and how to proceed. Computers do not fix cars, qualified technicians do. Being a good technician is not easy; if it were, anyone could do it, but it seems that does not deter everyone from trying!