The engine control module (ECM) communicates information through the data link connector (DLC). This data is transmitted at a high frequency which requires a scan tool or PC based scan program in order to read. With an understanding of the data that the scan tool displays, and knowledge of the circuits involved, the scan tool can be very useful in diagnosing a condition. A scan tool does not make the use of diagnostic procedures unnecessary, nor does it indicate exactly where the problem is occurring in a particular circuit. Diagnostic procedures contain information that can only be obtained with a scan tool or PC based scan program.