OBD-II is an improvement over OBD-I in both capability and standardization. There is a pin in the connector that provides power for the scan tool from the vehicle battery, which eliminates the need to connect a scan tool to a power source separately. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. However, some technicians might still connect the scan tool to an auxiliary power source to protect data in the unusual event that a vehicle experiences a loss of electrical power due to a malfunction. The OBD-II standard specifies the type of diagnostic connector and its pin out, the electrical signaling protocols available, and the messaging format. Manufacturers may also add custom data parameters to their specific OBD-II implementation, including real-time data requests as well as trouble codes. Finally, the OBD-II standard provides list of standardized DTCs. As a result of this standardization, a single device can query the on-board computer(s) for these parameters in any vehicle. OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter: P for engine and transmission (powertrain), B for body, C for chassis, and U for network. Most manufacturers have made the OBD-II Data Link Connector the main connector in the vehicle through which all systems are diagnosed and reprogrammed.
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