Abstract: To detect the failure of electronic control unit (ECU) during the product development and to diagnose its faults in the process of use, a testing and fault diagnosis system based on MC9S12DP512 for a certain AMT electronic control unit has been developed. The system can simulate various input signals and actuators during operation. Through the comparison between the feedback value and stimulation value, the detection to each module can be achieved rapidly. Experimental results show that the operation of this system is accurate and reliable. By using this system, it is realized to inspect and approve the performance of ECU and to diagnose its faults for the electronic control unit of specified AMT.