On your model car this is a common problem. Any time the light is on the system isn't operative. The exact problem must be determined with a scan tool. Normaly the cause is one of the seat belt buckles. Takes about 1.5 hrs to remove seat & replace buckle.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR