There are many types from just as many manufacturers, one of the most familiar being the Snap-On "brick", or MT2500/MTG2500.  Snap-On, OTC/SPX, Vetronix/Bosch, a number of other companies produce various types of auto diagnostic tools. from simple code readers to highly capable bi-directional computers with programming capabilities.

The vehicle diagnostic 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; Actual auto scanner  will display live data stream (inputs and outputs), have bi-directional controls (the ability to make the controllers do things outside of normal operations) and may even be able to calibrate/program modules within certain parameters. However, a typical car diagnostic tool does not have the ability to fully reprogram modules; This requires a pass-through device and specific software.

Through establishing the communication between automobile fault diagnostic computer and automobile computer, get shared data out from RAM of the vehicle computer microcomputer to analyze vehicle trouble, so the data stream testing function is an important function of car computer scanner. Data stream is divided into static data (KOEO) and dynamic data (KOER).

All vehicle that adopting microcomputer fault self-diagnosis system can using computer tester to readout the data stream that output by vehicle various electronic control unit microcomputer through diagnostic socket and reflects the electronic control unit controls system working conditions. Maintenance personnel can judge whether the electronic control devices work properly by making numerical analysis of various parameters of data stream, providing the basis for finding cause of the malfunction.

Parameters in data stream has two forms, numerical parameters and status parameters. The numerical parameters is the parameters that has certain units and a certain change range, which usually reflects the operating voltage, pressure, temperature, time, speed, etc. of each part in the work of the electronic control unit; state parameter is those who have only two states, such as on or off, closed or disconnected, high or low, yes or no, etc., it usually signify the working condition of the components of electronic control devices such as switches and solenoid valve.

In conducting numerical analysis, you must first distinguish between the various parameters are input signal transmitted to microcomputer from the sensor of electronic control device or the output command sent to the electronic control unit actuator by microcomputer. Input signal parameters can be state parameters, so can be numeric parameters. Most of the output command parameters are state parameters, a small part is numerical parameters. Data stream parameters can be classified according to various systems of vehicle and engine. Parameters of parameters of different types or different system have different analysis methods. During electronic control unit failure diagnosis parameters of several different types or different systems are needed to go through comprehensive contrast analysis. Different brands and different models of vehicles, the name and content of their electronic control device data stream parameters are not exactly the same.
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