after the control head goes through the calibration sequence, it enters diagnostic mode. Chances are very good that the temperature slider in the control head is bad and that can be tested with the diagnostics. It will flash the led slow for cold and faster as you go to hot. If you move the slider to hot and it flashes slow, bad control head. You have to step through all the diagnostics, it will test the mode ****, temp slider, etc. It's been awhile but when cal. is done, it flashes codes with the top led, then you push one of the buttons to move to the next mode, there are 4 or 5 different modes.