A T4 should tell you which of the 4 sensors is causing the light to come on. It wont tell you if it is the sensor, the cable or the magnetic ring though. It could even be an ABS main unit fault
Pretty comprihensive explanation of the Air bag light already given unless the car has been in an accident and the airbags have been deployed at some time, then the crash data would need to be removed from the main airbag unit.