05 Tahoe, auto climate control. Just purchased in late July. Today temps went down to mid 50’s, so I figured it would be a good time to test the heat out. Well, it only blows hot/warm at the max, 90* setting. Even if I put it to 89*, it starts to blow cool air. Set both sides to the same temp, or on different settings, it’s the same thing. Is this common? I would say heater core flush, but why would it only work on max temp? A/c blows cold, no problems there.