The dealer or someone else who can test the air bag system to determine what is at fault. I am a firm believer in airbags (tried them out once or twice, nice, very nice) and the whole system has to function properly for them to do their job. Usually it is just a sensor that is bad (the systems are actually pretty simple if you look at them) and they aren't that bad to replace. Try unhooking your battery for 2 minutes and then hooking it back up. You never know, it may just be a hiccup left in the system from a bad day.