Why my 06 nissan altmi turns over but will not start?

I was told that it may have been the fail safe (alarm system) I have now learned that, according to Nassian it was the cam and crank sensor. I was told that the dealership only has that part. The repair cost was $270.00. The sensor is located on the passenger side wall near the front window. All we can do is go by what they tell us. If there is another way, please let me know.

Roy Edwards
September 2010