The car service business has changed greatly over the years, largely paralleling the incorporation of new technology in every facet of our lives.
For example, at one time if an engine ran poorly, a tune up would usually solve the problem. No diagnosis or inspection was required. Tune ups are a thing of the past and no longer required on modern vehicles.
If a modern car runs poorly there could be dozens of different causes. This makes finding out what is wrong vital before making a repair; and the only way to find the problem is through diagnosis which involves: a skilled technician, a variety of specialized equipment, detailed vehicle information and sometimes specialized on-line services to assist with the concern at hand. Simply put, while diagnosis has a cost, it is money well spent because it allows us to find and fix the concern properly, first time around.
Guessing at the cause can be very costly and ultimately frustrating for you, the car owner. Occasionally there is an assumption that auto service technicians will just know what the problem is. While we may have hunches or guesses, they could be wrong and diagnosis ensures that the real problem is found.
A good comparison to illustrate this point is that, just like a doctor might have a hunch that chest pains could be a heart problem, he will seek out diagnostic tests before making erroneous conclusions.
On a modern car, most problems require diagnosis before a repair, owing to advanced electronics along with very tight and complex parts assemblies. Overall the value of diagnosis is finding the problem first time around. This saves you time and money while giving you the confidence that your car is fixed properly.