| The improvement of the efficiency and accuracy of the engine diagnosis is strongly required by the by the increase of the preservation of cars and the emission-related rules which becomes more and more strict. One of the effective solutions is expert system, while the on-board diagnosis (OBD) is another. This paper studies the cooperation of both technologies and tries to develop an engine diagnosis expert system based on OBD information.This paper analyzed the basic theories of the OBD and expert system, defined the functions and requirements of the diagnostic system. After that, the structure, the basic work flow and the communication protocol of the diagnosis system were designed and the hardware platform of the diagnosis system which is based on an MC9S12DP256B microcontroller was developed. With the basic types of engine malfunctions analyzed and the actual process of the engine fault detection studied, the methods of knowledge expression and reasoning were finally determined to carry out the diagnostic strategy.According to the characters of the hardware platform of the system, the page organization method of system functions and the button-event-drive method were accepted to multi-use the buttons and to improve the system capability of managing the interrupt routines.The basic functional modules, basic missions and work flow of the diagnosis procedure were designed based on the analysis of the modes and services of the system. As a result, the software platform was developed, and the debug environment was established to test the hardware and software. The Result of the test showed that all the designed diagnostic functions worked properly and the design requirements were met.
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