I have a 2007 Volvo S60 2.5L. So for years my heat would blow cold air intermittently and I had asked my mechanic to check it out but they couldn’t find anything. I didn’t really press the issue so I’m not sure how thoroughly they checked. NOW my heat ONLY blows cold air and my temp gauge on the dash is at the 12 o’clock position. My coolant is full. What could be the problem???