With the introduction of computer controls, there is a greater need for optimum performance by all the components in the system, including associated sensors, actuators, and other engine components. If even one sensor is out of calibration, it can upset the entire balance of emissions and fuel economy. An engine component failure such as a burned valve can cause major changes in engine operation. The excessive hydrocarbons can overheat the catalytic converter and the extra oxygen will drive the feedback system rich. That is why all parts must work together to achieve peak performance.
Performance is the operative word when discussing a modern tune-up.
They do not understand things like closed loop fuel control, electronic spark timing, or idle air control systems, but they do know when their car is not behaving the same as it used to. Symptoms such as cold start problems, surging, poor fuel mileage, and dying at stops are just a few of the items that will cause a customer to come in and ask for a tune-up. However, its meaning varies from one person to the next. Technicians generally think of a tune-up as replacing spark plugs and checking timing, idle speed and idle mixture. A customer may not think of a tune-up in the same way.