I have a commercial True Z 8.1 treadmill that I received second hand. It has been working ok until today when I have an err message that says Key Stuck Grad- on the display. I have tried looking on the internet to get information and I can't find anything. Does anyone know what this err means and/or how to fix it? Is this an inexpensive fix or more expensive problem? Any help with this matter would be greatly appreciated. Thank you.