Scan tools are used to run diagnostic tests on vehicles to obtain information on the probable cause of faults. They are able to communicated with the on-board control computer fitted in a vehicle.
A scan tool is a device that is able to communicate with the on board control computer fitted in a vehicle. The scan tool is a software driven device that contains information about the particular vehicle it is fitted to and communicates with the various on board systems to determine their status and condition. A scan tool can be a standalone battery operated piece of specialized equipment, or it can be a device that links the onboard computer system to a PC running 'onboard diagnostic' software.

The automotive industry introduced 'on board diagnostics' standards OBD1 and OBD2. The original OBD1 specification did not have a standard link connector, nor a standard data protocol, so the early on board diagnostic systems were highly proprietary to each manufacturer. The OBD2 specification standardized the connectors and the test signaling protocols, and also specified a common but extensible set of Diagnostic Trouble Codes (DTCs). OBD2 started to become mandatory for
all vehicles in 1996.

The scan tool connector is usually located in the front of the passenger compartment on the driver's side, so that the technician can operate the scan tool and see any Malfunction Indicator Lights (MIL) that may be on the instrument panel at the same time. The scan tool displays the Diagnostic Trouble Codes that have been stored by the vehicle's control system, and it can also carry out some diagnostic tests on vehicle systems. The intention is to provide the technician with enough diagnostic information to be able to identify and rectify faults. When the faults have been rectified, the scan tool can also reset or clear the DTCs.

Here is good OBD2 scan tool for you - This OBD2 Scanner