The Air Bag is referred to as a Supplemental Restraint System (SRS) component because it is designed to work with, or as a supplement to, the seat belts supplied with the vehicle.
A basic SRS is comprised of the following components:
The SRS Sensors are used by the SRS Control Unit to detect a moderate to severe frontal collision. If such is the case, the SRS Sensors send a signal to the SRS Control Unit, which in turn activates the SRS Airbag. The SRS Control Unit also has it's own emergency backup power in case the vehicle's electrical system is disconnected in a crash.
The SRS Control Unit also monitors the integrity of the SRS System once the ignition key is switched ON . Under normal operating conditions, when the ignition key is initially turned to the ON position, the SRS Indicator Light should come on for 6 seconds, and then go out. This serves as a bulb check for the SRS Indicator Light. If the SRS Control Unit detects a problem in the SRS system, the SRS Indicator Light will remain ON until the problem is resolved and the SRS Diagnostic Trouble Codes (DTCs) erased.
The SRS control unit has a fault memory, which can be accessed using a suitable Diagnostic Scan Tool (DST) such as the VAG 1551 or VAG 1552 connected to the 16-pin Data Link Connector (DLC).