You cant turn the airbag off per say, apart from disconnecting the plugs, If its not the dash bag (that someone has disconnected cause of using a baby carrier) it may be the seat airbag, its quite a simple fix... Its probable that the previous owner had something rolling around under the seat and its just knocked the seat plugs... Simply disconnect battery, unplug and replug the seat connector, connect battery and then if you need to do a battery reset....