BMW Z4 airbag light on
Z4: how do you read the airbag light fault codes?
Through the OBD connector the car gives access only to engine and transmission control. The airbag control unit is read separately with BMW's own diagnostic software. A cheap reader made for Japan does not suit a European car. The easiest way is to have a workshop read the codes.
Z4: why does the airbag light come on again after the fault codes are reset?
A reset clears only the codes in memory. If the fault is still present, the light comes on again. The most common cause is a bad or oxidised connector, for example under the seat or at the B-pillar sensor connector, and moisture can cause it.
Z4: why did the airbag light come on after the door was removed?
The door wiring harness connector may be losing contact, in which case the control unit logs faults in the side airbag circuit, for example an open circuit and too low a resistance. Replacing the connector fixed the light. Have a workshop do the check and repair.