In this paper, a multi-functional control module is developed to improve the confidence of $CO_2$ welding. In the developed system, a main controller of an automatic welding system including I/O port is modulated by microprocessor. The developed multi-functional control module is familiarized to the conventional automatic welding system. Therefore, the confidence of $CO_2$ welding is improved when the main controller of an automatic welding system is broken down.