In the early 90's monumental changes took place in the fueling and ignition systems on modern motor vehicles.
 Gone are the days of the carburetor for fuelling the engine and the old contact breaker (points) for making and breaking the primary circuit, which in turn contributed the ignition of spark at the spark plug.

Modern vehicles have both ignition, and fuelling controlled by means of a computer (ECM).
When a fault is detected in the system, it is stored in the ECM, which in turn triggers a (MIL) llight in the dashboard.
As soon as the fault has been rectified, a diagnostic serial tester is used to erase the fault code and extinguishes the warning lamp indicator (MIL).