See also:

Office
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Functional readiness of the airbagsystem
When the ignition is switch on, the warning lamp in the instrument cluster lights up briefly and thereby indicates the operational readiness of the entire airbag system and the belt tensioner. ...

Safety belt reminder for the driver's and front passenger seat
The indicator lamp flashes or lights up and a signal sounds. Make sure that the safety belts are positioned correctly. The safety belt reminder is active at speeds above approx. 5 mph/8 km/h ...