The only reason I can see that Hertz could/should have asked the question is if the OP had booked a group A or B car and had been upgraded in the Carfirmation. Even then, with the faulty airbag light, this should have been a complete non-issue assuming that the car actually was GC and the OP is not mistaken and the OP's allocated car could have made up the space in GC.

Heck, even if the OP had taken someone else's allocated Mercedes from the spot next to his, the agent should have done what he could to accommodate OP (by which I don't mean give a free upgrade to anything on the lot, but offer a reasonable rate or a working alternative car if that particular car needed to go out on an allocated reservation).

OP - if the cars in GC were crap (and they often are) why didn't you check out the Carfirmation alternatives?

Finally, when I was looking at GC cars at LHR a couple of weeks back, the attitude from the agent was "what are you doing here/" rather than trying to sell one or you know, help me...