Every now and then when I go to crank up my car it will just shut off. After I wait a few minutes it cranks right up. I noticed the check engine light came on for about two days so I took it to my mechanic. He hooked up the machine to see what the problem was and I believe it said P-340 (camshaft position sensor a bank 1). Where is this sensor located and how hard/expensive is it to replace? Any suggestions would be greatly appreciated.