Temperature Gauge

I have a 2001 Mit. Galant and recently my temperature control for setting my air has gotten difficult to turn. I can move it to the hot position but I only get cold air and no heat when I turn it up. My air conditioning is very cold and stays cold when I move the temp. gauge to hot. I am just wondering if anyone knows if this a minor control gauge problem or a more serious problem, such as the heating unit itself. Whatever anyone could offer would be great.