My 2003 BMW 745i has a fault in the passive restraint system for the driver side head curtain airbag. Dealer says the whole airbag thing needs to be replaced at a cost of $1500. There was no damage to the car - airbag was not deployed. Why would it just “go bad”? Another mechanic found other faults on the computer and suggested maybe one of them caused the fault in the airbag. Should I replace the airbag, or suggest to the dealer that other faults may be at fault? Is this a common problem in the 2003 7 series?