My car starts and after about 15 seconds it stalls out. Why???

Just because you get a code like a P0340, that doesn't mean that the sensor itself has to be replaced. The code means that the PCM doesn't like the signal that it is seeing. That could be a power or ground circuit issue for the sensor, the wire in between the sensor and the PCM, the sensor or the PCM themselves, or it could be a synchronization issue between the crankshaft and camshaft sensors (timing belt?). Someone will have to test this and prove why the code is being generated and that will mean using a volt/ohm meter or oscilloscope.

AGREE WITH #1 TO MANY PEOPLE THINK YOU JUST CHANGE THE PART AND ALL WILL BE GOOD. CAR NEEDS DIAG BY A MECH