Presented in this research is methodology for modeling the behavior of an automotive ECU (Electronic Control Unit) to verify in a production system. The methodology of this paper is to verify the defects of ECU products during in a real operation, before the defective ECU equipped to an automobile. The performance of an ECU operation is dependent on not only the specification of a hardware device, but also a software program installed in the memory of an ECU. The software program of an ECU is able to be validated before installation, but the validation process is usually executed in a very controllable environment. In order to consider the software program which is frequently changeable in practice, the verification methodology of ECU products as the hardware device and software program integrated is required for detecting defective ECU products caused by inappropriate software program during in manufacturing processes. The main objective of this research is to suggest the modeling methodology of ECU behaviors as the input of a test system. In order to model the system input, this research acquires the data of signals from sensors and loads on actuators, during the operation of an ECU product equipped in a real vehicle. The acquired data is analyzed to generate a model that imitates the behaviors of an ECU in operation. The provided modeling methodology has been implemented and tested with several products.