Airbag fault could be a serious safety issue if not resolved. There a a number of components related to the airbag starting with the controller unit, this unit sends and receives information to the vehicles on board computer letting it know the 
health of the system. The airbags themselves will repot to the controller about the status of the airbag and its trigger mechanisms, if anything is out of specification it will register an error.
If some work has been carried out on a seat with an airbag or a steering wheel an error could be reported even though the component has been replaced correctly.
By running a diagnostic on the airbag system it is possible to correctly identify where the problem lies, it may only need to be reset to resolve the issue.