Snap-On, Hella Gutmann Solutions, OTC/SPX, Xtool india, Autel, Launch, Vetronix/Bosch and a number of other companies produce various types of scan tools, from simple code readers to highly capable bi-directional computers with programming capabilities.
The scan tool is connected to the vehicle's data link connector (DLC) and, depending on the particular tool, may only read out diagnostic trouble codes or DTC's (this would be considered a "code reader") or may have more capabilities.
However, a typical scan tool does not have the ability to fully reprogram modules because it requires a J-2534 pass-through device and specific software.
The tool communicates with the vehicle's onboard computer and retrieves the DTC codes.
The codes are then interpreted by the mechanic or technician to determine the specific problem with the vehicle.