This paper aims at developing the offline Simulink model for different diagnostics algorithm for BS-VI On-Board Diagnostics (OBD). Union Government has announced that it will enforce the BS-VI emission norms for automobiles by 2020. BS-VI have stringent norms compared to BS-IV along with a Real Driving Emission (RDE). Thus, it introduces new methodologies based on estimators and physical modules for different subsystem interfaces, such as Air handling, After treatment system, filtration system, etc. This is a challenging problem to develop robust diagnostics due to increase in the number of complex electronics. It requires strong tools for the analysis of vehicle duty cycle and testing cell data, which will help to decide on tuning strategies. In such scenario, offline simulation tools will be very helpful and cost effective in tuning and validation process of diagnostics and features. This paper explains the offline simulation diagnostics methodology for various BS-VI applications with virtual sensor concept instead of using an expensive physical sensor, which in order helps in reducing the cost of the overall system as well as the tuning time in test laboratories.