Lisez vos docs, bon sang !
"Disable Mode
When playing online games it is often necessary to disable the modchip so you do not get a DNAS error while connecting to the online game server. To disable DMS4, hold down L1 on controller 1 as the PS2 is booting. You will see a message saying that the modchip has been disabled. Insert your game and reset the console - the modchip will be disabled and will stay disabled until the PS2 is placed in standby."