BMWI 2000 Sedan 45,000 mi. No.52 engine

Which cam sensor did you replace, the intake cam or exhaust cam? Did you clear the faults and reset the adaptations? Where did you obtain the sensor? Who was the manufacturer? The P-code P0340 is a nondescript code generated by a generic scanner so if you replaced the exhaust cam sensor then you need to go back and obtain the intake cam sensor and install it, then clear the faults and that should do it.

If you replaced the intake cam sensor and the fault came back then either the exhaust cam sensor needs to be replaced as well or the sensor that was installed is faulty. How are you obtaining the fault code? If you are reading the faults with a generic scanner, then it may not pick up that there is a distinction between the intake and exhaust camshaft sensors. Resetting of the adaptations can be performed by the more sophisticated aftermarket scanners or the BMW proprietary software(used at the dealership and referred to as the GT-1) and may not be needed in your case. The cam sensors do fail especially if they are original equipment. I suggest replacement of the exhaust cam sensor, try realoem. Com for BMW parts online, then clear the faults and try it again.