Mercedes-Benz Of Reno

I brought the car in for an oil change at 11:40 appointment. I waited for the repair at the dealership. At 3:20 I was told it was completed. But was told it would be completed much earlier. I was told there would be no charge for the service. I appreciated that but was surprised since they did the work. I asked for a receipt of the service, but was told I did not need it. I said I would like it for my files, but told it was in your computer system. I would still like the paperwork. At this I question why I could not get a copy of the work done. This makes it difficult to answer the questions below.