I fixed my airbag light by buying another airbag sensor module. Mine had went bad, the shop I use did the diagnostic for free. The module itself cost like $1200, on ebay its like $70.

If you do change it, the thing is located underneath the center of the dash on the floor.

Its in held in with 3 screws. Once I swapped mine out with the ebay one light went out. I think the just go bad and have to be replaced sometimes.