Diagnosis is an algorithm for finding and isolating faults in a dynamic system. In 1994, California designated some regulations which were called OBD II. According to these regulations, there is a system installed in an automobile which can analyze the function of the automobile continuously. The decrease of pollution for the expansion of diagnostic system is necessary in the future. To reach the aims of diagnosis, some redundancies are required in the system, either hardware or soft ware. In

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... e hardware redundancy methods, the installation of additional sensors or actuators on the system is required which is costly and takes up a lot of space, whereas in software redundancy methods, this is done with no expense. In this article, one of the software redundancy methods or analytical methods is implied for solving the problem. At first a discussion on literature survey is mentioned, and then a modified mathematical model for SI engine is acquired. The usage of this method and parity space relations, which is a model based method, accomplished the process of diagnosis. Developing a modified SI engine model and diagnosis of MAT sensor which less has been considered besides other components are this article contributions.