for the HP EliteBook 840 G6 that works on all devices. HP uses a security chip (Nuvoton NPCE388N or similar) that stores passwords in write-protected flash, and newer firmware prevents simple software overrides.
Would you like a detailed guide on the SPI flashing method (if you're technically inclined) or help getting the disable code from your locked screen? hp elitebook 840 g6 bios password reset utility work