If I can't trust you to service my car how can I t...

If I can't trust you to service my car how can I trust you to sell me a new one. They sent me a coupon for a free oil change in the mail. I went in to get my free oil change and when I left I noticed the airbag warning light was illuminated on my dash (I have never seen this light come on before, I have a 2011 Mazda 3 that never needed a repair) so I return to service to have them look at it. They tell me the airbag sensor in the passenger seat needs to be recalibrated and it cost $100 or if I don't want to fix it the inspection fee is $100 so ether way my free oil changed turned into a $100 profit for them thanks a lot. I talked with the service manager and he didn't care. Now I'm thinking they just unplugged the sensor when I came in for the oil change and plugged it back in when I came back and charged me $100 what scam. Dear Chad, the information you have is wrong. I returned the next day after the oil change although that visit might have been "off the books". By the way how does one recalibrate an air bag sensor?