i get the p0340 code for camshaft position sensor

Hello, The P0340 code does not mean the camshaft position sensor is faulty, but that the ECU is not receiving the signal when the vehicle is cranking or started. This will cause poor performance, stalling, or failure to start, as you have noted. Since you have replaced the sensor, ensure the connections are all clean, and that vermon have not chewed any wires before you attempt to diagnose the ECU. There is a loss in signal somewhere between the ECU and the sensor, and sometimes this can be caused by batteries or even a bad starter pulling too much electricity during starting. Ultimately, you may have to bring this one in for diagnostics, and a BMW/German auto specialist will likely save diagnostic time/cost. See the link below, and if you do not have one in your area, use one from the list that has ASE or factory certified technicians. http://repairpal.com/bmw-within-10-miles-of-new-britain-connecticut-06051