I bought an 04 A6 2 weeks ago. It needed new rotors on the back. I took it in and they put new rotors on the front. The back rotors are visibly grooved. Should the back rotors be replaced for free as well? The car stopped doing all the vibrating and pulsating like it was doing before i took it in to get it fixed the 1st time. But the back rotors still need to be replaced. The car was certified when I got it. Arent the rotors on the check list?