More than weeks to repair a vehicle

I purchased a VW Scirocco form McCarty Nissan gateway. The vehicle was delivered to me on the 12th of August. The vehicle had a miss on the engine and the salesperson said they washed the engine and that it would go away. The second time I used the vehicle the catalytic convertor light came on. I took it to VW for a services that was due and they reported that the vehicle had a compression problem. I contacted Nissan and they collected the vehicle on the 12th of September from me and took it to be repaired. It is more than five weeks since they took the vehicle from me and I still don't know when I will get the vehicle back. At this stage I wish I could just cancel the deal.