These days more people are aware of the global warming and CO2 emission, governments are imposing strict standards for Automotive manufacturers to follow. One of the standard which is widely in use in the automotive industry is OBD-II which defnes the vehicle's self diagnosis and reporting capability. It provides a list of vehicle parameters to monitor and how to encode the data. Here DTC or Diagnostic Trouble Code tells the fault which is detected in the self diagnosis.

Modern vehicles are equiped with more electronic parts which gives it intelligence and better capabilities. A car might contain 10s of microcomputer units inside it which is called as ECU (Electronic Control Unit). These ECUs can take input from the sensors and act upon those information and can also control some actuators(Depending on the function of the ECU). For example in Electric Vehicles there will be a separate ECU for monitoring the Battery pack and separate ECU for controlling the charging.

Now these ECUs are interconnected using a network called as CAN (Analogus to ethernet in computers). ECUs passes information between them in the form of CAN messages. 

Whenever the vehicle is in operation, the ECUs keeps doing self diagnosis. Self diagnosis includes checking of the sensors, input signals, communication, on boards devices etc. Results of the diagnosis will be saved in the ECU, depending on the importance of the unit for which diagnosis and the type of the unit and ECU data sotred will vary. Each diagnosis unit in the velicle will be associated with a unique identification code of 3bytes called as Diagnostic Trouble Code.Conventions are followed in associating a DTC with a diagnosis unit.

OBD Data Link Connecotors are used to connect an external scan tool to the vehicle, usually located under the dashboard. The scan tools can now access the CAN netowrk and then communicate with the ECU. Lot of scan tools are available in the market. Scan tools uses special protocol to communicate with the ECU(KWP 2000, DiagOnCAN etc)

These scan tools will search all the ECUs for the stored diagnostic results. ECUs will send the DTCs for which the some problem was detected.