This paper presents a system concept of the On-Board Diagnostics system (OBD-II) in the Hyundai Accent. New-α and α-DOHC engine developed by Hyundai are installed in the Accent. The ECU (Engine Control Unit) developed by BOSCH is adopted for this vehicle. To comply with the OBD-II regulation mandated by CARB (California Air Resources Board), some monitoring algorithms originally developed by BOSCH were introduced and modified for the Hyundai Accent. Using modified algorithms, many kinds of test were carried out during more than four years. Through the demonstration test and various field tests, it was confirmed that the OBD-II system fulfilled the regulation and had good performance.