05 S60 Interior Heat Problem

[Minneapolis_Mandy]

I have a 2005 S60 2.5T AWD. It is approaching 100k miles. It has manual climate controls. It has recently stopped blowing out heat while when the "Auto" button is on (and the A/C is turned off). If I turn the knobs from hot to cold still with the A/C off, it suddenly blasts out cool air. The car will still blow out heat without the "Auto" button on if one of the other buttons which direct air to the feet, midsection, etc., is on. I have always used the interior fan on "Auto" for both heat and A/C, and when I want heat I just turn off the A/C button and turn the heat knobs up. It really used to pump out the hot air that way. I'm wondering why the heat has stopped coming out on the "Auto" setting with the A/C off. Any help would be greatly appreciated, its getting cold in Minneapolis!