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Congress passed the Clean Air Act in 1970 and established the Environmental Protection Agency (EPA). This started a series of  emission standards and requirements for maintenance of vehicles. To meet these standards, manufacturers turned to electronically controlled fuel feed and ignition systems. Sensors measured engine performance and adjusted the systems to provide minimum pollution. These sensors were also accessed to provide early diagnostic assistance.  The current late model OBD-II emission system is an expanded set of standards and practices developed by the Society of Automotive Engineers.

One of many components may be causing an engine to perform below standards.  State of the are diagnostic computers may be necessary to determine problems.  Diagnostic computers can read codes that are stored in the vehicle computer, and also measure current engine performance.  A trained technician can interpret the data retrieved from the vehicle computer and then recommend repairs.

Diagnostic computers are expensive pieces of equipment and a trained technician is an investment.  Consequently, minimum diagnostic fees may be assessed when troubleshooting drivability problems.