Side/window Air Bag problems of the 2008 BMW 535

At c. 89,000 miles - warning: "driver side air bags malfunction". BMW dealer, rockville, MD, wants $660 to tell me if it's a real problem with the bag, and/or its housing and/or only a faulty sensor sending an incorrect message (!!) following that suspicious drill they can then fix it - a couple hundred dollars if it's only a sensor, much more $ if it's something else. Car moving or not, the warning remains on.