What they do:
The Scan Tester family of products provides enhanced capabilities to communicate with ECUs onboard the vehicle, especially with the Onboard Diagnostics functions (OBD functions) of the ECUs. The products currently available in this family include the MTS 3200 (Consult II), and the Tech 2.
Connecting a Scan Tester to a PC allows for data display, storage, and printing in addition to ECU reprogramming and software updating of the Scan Tester.
Why you need them:
Scan Testers are needed in order to access the ECUs on-board a vehicle so that testing and diagnosis can be accomplished. Scan Testers enable many special tests and provide a user interface to the vehicle's on-board diagnostics.