On Board Diagnosis costs money. If OBD is not considered early in the design process, it may cost a lot more money. This presentation covers the conversation of design systems for OBD Diagnostics including OBD consideration in all design stages to allow for an optimal cost-function trade-off in Hardware selection, Sensor architecture, SW design and Calibration of function. Finally this presentation will address why the best design for OBD is often not the vest design for function.