Nowadays, FPGAs play a significant role in industrial applications. Therefore, ensuring their proper performance is of great importance. In this paper, an automatic Built-in self-test core has been designed in the LUT to test and investigate errors. In order to verify this method, powerful software H-Spice with 45 nm precision in transistor level has been used. The advantages of this method include being automatic, detecting various errors, preserving initial information, and reducing hardware overhead compared to the previous methods.