1. An apparatus for diagnosing an internal combustion engine including a plurality of engine components and a plurality of engine operating parameters, said engine having engine specifications for said engine operating parameters, comprising in combination: input means for providing input signals representative of said engine operating parameters; memory means for storing said engine specifications in the form of discrete coded data specifying a problem level associated with one or more of the operating parameters; processor means for: (i) respectively comparing said input signals and said engine specifications; (ii) generating a plurality of diagnostic signals, each of said diagnostic signals representing the correlation between a said engine operating parameter and a said engine specification; (iii) selectively analyzing at least one of said diagnostic signals; and (iv) generating an instructional signal indicating an operating state of at least one of said engine components; and display means for visually displaying said instructional signal.