To complete a smog check in California, you typically need your vehicle registration and, if applicable, the DMV renewal notice you received for vehicle registration renewal. These documents help the smog check station verify your vehicle information and ensure the test results are properly reported to the appropriate authorities. Many modern smog check stations can also access vehicle records electronically using your license plate number or Vehicle Identification Number (VIN), making the process faster and more convenient. However, bringing your registration documents is still recommended to help avoid any delays during the inspection.