2005 chevy monte carlo
 

My monte carlo will not start just clicks. Took to mechanic cost me 250 to do electrical diagnosis. And then he said was just battery and needed to reset computer. Went to get car and it started once, cut off and would not start again. Now he is telling me it is camshaft sensor? Does he know what he is doing ? Any suggestions cause now he wants another 450 dollars to fix?