As chris said check the level sensor. if this is attached then whilst you are under there check that the tubing is well seated in the shocks (push then in as far as poss and then give a little tug). If all good there then try a reset. To do this turn the system off using the switch in the cubby hole in the boot, then jack up the rear so both wheels hang to their full extension and leave for a couple of minutes. then lower back down and switch back on.
HTH