Domestic vehicle manufacturers designed their computers to have an accessible data line where a diagnostic tester could retrieve data on sensors and the status of operation for components.
These testers became known in the automotive repair industry as Scan Tools because they scanned the data on the computers and provided information for the technician.
The Scan Tool is your basic tool link into the on-board electronic control system of the vehicle. Scan Tools are equipped with, or have separate software cards, for each OEM needed to be diagnosed. In this case, always secure a scan tool that has the latest OEM-specific diagnostic software included.
DaimlerChrysler specifies the use of a DRB-II or DRB-III scan tool with its diagnostic processes. However, there are aftermarket scan tools, that when equipped with the right software, can provide proper diagnosis as well.