The code is based on the service tag of the system - if the unlock did not work, there's a fault with the mainboard.  That's why I asked if the board had ever been replaced - if it hasn't and the unlock does not work, the board is faulty and will need to be replaced.

It may be the EEPROM chip failed and is corrupted -- but you're likely to find the cost to have that unsoldered, reflashed and reinstalled to be as much or more than the cost of a replacement board.