The air shock sensor quit working at around 50,000 miles (4 years old). This caused the rear end of my Armada to sag and bottom out when driving over just about any bump or pothole. This must be a fairly common problem with the vehicle, because my mechanic said it was a very hard part to find. All the dealerships in my state were out of the replacement sensor. We had to have one shipped from a dealership in another state (luckily at no additional charge). I've read about this problem all over the message boards. Seems like a good candidate for a recall.