What wrong with my treadmill?

I recently got a used Free Spirit Treadmill. When I turn it on the board lights up and flashes on Program I. After some fighting with the buttons I am able to switch to other programs, but it will not let me use the incline +/- or the speed +/- buttons. The treadmill will only work if I hold the start button down once I let go of it the belt stops moving. I have checked the wires and connections and everything looks good. Does anyone have any idea as to what might be wrong with my treadmill. I would love to be able to fix this without having a service tech come in. Thanks Sheri