I'm guessing the the one's they keep changing is the newish one's with a key Pad? ( from September 2004)

parameter 39 locks the can system:
1 = Ignition, open
2 = Ignition, locked
3 = Keyboard, open
4 = Keyboard, locked

Set the parameter to 1 or 2 if only three drivers/truck profiles are to be programmed and a key is used to start the truck. If using a keyboard and up to ten drivers/truck profiles are to be programmed, set the value to 3 or 4. With the latter, the value must be programmed for each driver.