I have a red airbag light lit up on my dashboard and its been like that a while. I took my car into Audi for some quick service and while it was there they scanned it over and said that the airbag is defective and needs to be replaced at the tune of $900.

How does a airbag go bad? Is this just a sensor I can replace? If not, what are some cost effective options to replace the air bag, since I plan on selling my car soon?

--Nick