Since the late 70's, automobiles have been inundated with emission and computer controlled devices for the auto technician to diagnose and repair. In most cases, a faulty component will not show signs of physical damage. Sometimes a failure of one subsystem might act as though some other part may be at fault. It is essential that the technician understands the operation of each component before the system is deemed faulty. Always check the integrity of any new parts installed.
After gathering the background information from the customer, a diagnostic scan and a road test should be the next plan of action. Always record any trouble codes before clearing the ECM memory, and record any codes appearing after the road test. If you develop a standard routine for asking questions, the better chance you will have of solving the problem faster and less expensively. Seems like we should have been doctors, eh!