Bmw m3 e46

Bmw m3 e46 

My son’s 03 M3 has had the SEL activated three times with resultant code P0012 (‘A’ Camshaft Position Timing Over-Retarded (Bank 1)). Each time he gets the SEL the engine idles and runs rough. However, the first two times after the code was cleared, the engine ran normally.Then after a few days, weeks, no particular time frame, the SEL comes on and rough running conditions exist again. The rough conditions begin upon an engine start. I have an 04 M3 and have not experienced these symptoms. Before he has the Intake Camshaft Position Sensor replaced (which is pretty expensive on the S54 engine), we were wondering if there is some other problem since once the code is reset/cleared, the engine runs fine for awhile.The BMW mechanic and the indie shops just say to replace the camshaft sensor. The mileage is 50,000 and there have been no modifications other than an aftermarket cat-back exhaust system. Thanks in advance for any help you might be able to give us.
A
The code that you mentioned could potentially be due to a faulty sensor or perhaps a VANOS problem.  Of these, the sensor would be more likely.  Early failures of the camshaft sensors is not uncommon.  Also, we see that there has been a supersession on the sensor, in 2006.  This is likely due to a redesign.  Before just throwing a sensor at it, you could install the sensor, from your M3, onto your son’s car and see if the fault continues or stays cleared.  This will tell you if the sensor is indeed at fault.  If the fault continues, you may have a VANOS unit problem