It could be as easy as a reprograming or all the way to a full system replacement. What you need to know is whether you had your car on a lift recently. x5's don't like to be lifted off the ground, it screws up the autoleveling sensors. An airbag will run you from 175 to 275, used / new respectively. Take it to a BMW specialist and have them run the error codes off the ECU, DON'T AUTHORIZE ANY WORK UNTILL YOU SEE THE ERROR CODES!!!! The air seuspension can get freeky expensive. That's why you go to a shop that is certified or a BMW service center. Just make sure to tell them not to do any work without your consent. Hopefully its just a reprograming. Let us know more info on how it came up and when you last got work done to it and we'll be better able to help you.