Remove one stick of RAM, boot the machine, then reset the PRAM (Option + Command + P + R).
Several third-party tools are available that can help you unlock the EFI password. Some popular tools include: unlock efi password
This guide explores what an EFI password is, the official ways to remove it, and what to do if the standard methods fail. What is an EFI/Firmware Password? Remove one stick of RAM, boot the machine,