newbie c5 owner with autobox problems.

[daveth]

hi all.i have a citroen c5 2.0 hdi auto.the problom i am getting is that when i drive the car for more than 100yds it stops driving and makes like a grinding noise.i have changed the oil and filter in the gear box.also its flashing automatic gear box error.does any one know what the problem could be plz. many thanx dave.