i had the same problem on mine,my freind is the workshop controller at audi so he fixed it.very common problem on audi a4s.if it is what he said was wrong with mine then your airbags will still work. there are two connectors under the drivers and passenger seat if you unplug them for a 5 mins and plug them back in again that cures the fault but the light will still stay on.you have to go to a vag dealer who plugs a machine into a plug on your ashtray and clears the fault code in your cars computer.luckly he had a handheld one at home in which he cleared it for me he said vibration causes the plugs to come loose.