Starting 2001 all vehicles, which correspond to the exhaust standard EURO III and EURO IV, are equipped with a standardized OBD system. The OBD supervises the overall system engine and pollutant-relevant parts. Thus errors are recognized and indicated by lighting up the error lamp. Frequently however not of the OBD indicated part is alone responsible for the disturbance. The actual cause can be because of completely different place. This brochure gives answers and tipps to the fault-tracing.