Abstract: A system and method which systematically diagnoses emissions test failure by applying the rules of a knowledge base to predict the cause of vehicle emissions failures. Classifiers are used to form predictions. The classifier is the data structure used in the automobile emission testing inspection lane by the lane diagnostic subsystem to provide a diagnosis for a particular vehicle. Its output is the likelihood that a vehicle suffers from a given failure based on the values of characteristics such as its emissions test results and the vehicle's description. The classifier predictions are then used to prepare a failure report that is given to the motorist for use by his or her repair technician. In another feature of this invention, the classifiers are continuously updated in a learning process based on new repair records. The learning processes periodically analyzes the data and updates the knowledge base to include new or revised classifiers.