I have a Kitchen Aid Microwave/ Oven combination. 3 of the touchpad buttons either don't work or work intermittently. I know the problem can be membrane, cable or control panel. When I push the button, there is zero response either visually or functionally. The other buttons work fine. Is it more likely to be one part more than the others? what is the best way to test them if I do not have a schematic diagram. Thanks for any response.