The heater temperature control was designed with a lever which clicks as it goes from cold to hot. A little ball on the end of a tiny spring bumps along a grooved bar giving digital feedback. When the ball falls out, the plastic piece which holds it catches on the little grooves and jambs. When you take the control apart, you will see all this. The easiest fix is to cut a thin piece of strip metal (I used a piece of 1/4" wide tape from an old tape measure) about 4" long. This is glued onto the top of the the grooved bar, covering the grooves so that the plastic piece doesn't jamb on them, but slides smoothly over the metal strip covering the bar surface as the control is moved.