Can anyone help me? I have a used 2008 Nissan Serena C25 which was overheating so we replaced the MR20 head and block with another used one. The overheating has stopped and vehicle was working fine until one day I stopped at the traffic light on an incline, and the vehicle eventually went limp and cut off. It gave trouble to restart and when it did, it did not want to move until I revved continuously releasing quite a bit of smoke and it sounded like something was choking the system. After putting it on a scanner, the following codes have been found: P0172, U1001, P0507, P1550, P0725, U1000, P1722. The AC also is not functioning properly. Any suggestions?