After a 1 day diagnostic I was told by Mazda dealership my 2004 mazda 6 i timing chain needed to be replaced because of a code P0340. First they said the timing chain was good. Then they said it had to be replaced. They were not confident that that was the problem. The camshaft sensor has been replaced. The tachometer reves from 0 to 3. Other than that the car runs good. I have gotten a second opinion from a mechanic who said it could be the variable timing valve. Any supportive help on this subject?