Could also be the lowering pot screw on the A5 card has moved.

If the sensors & height counter are working correctly then remove the arm rest panel, on the large silver card with all the green & red led's roughly in the middle is a screw potentiometer - this can be used to trim the lowering speed.

Turning it anti clockwise will increase the lowering speed, just turn it a little bit at a time until the lowering speed is correct.

Once I've set these up I normally stick a little dab of silicone sealant on the screw to stop it moving from vibrations due to rough floors.