The temperature control (Hot/Cold) on the HVAC panel is cable controlled and can become worn. Had the same problem on my 4th gen. Get in the floorboard on the passenger side and see where the cable connects to the blend door, underneath the dash and document holder. Then move the temperature control back and forth from hot to cold and see if the cable is pulling or closing the blend door.

Mine wasn't and was stuck in the middle position. I manually closed/opened it and got cold ac and hot heat. It's a quick, easy check.