This NordicTrack C2255 in Sanford, Maine would begin to run and then quickly shut down in one or two seconds. It was diagnosed as needing a new motor controller, also known as a motor control board.
The motor cover on this model has five screws that need to be removed for access to the motor and motor controller, etc.. Three screws are removed from the front. The other two are on the bottom at the back. To remove them you need to fold up the machine, then lay the treadmill down to access the two recessed screws. You will need a long #2 phillips screw driver to unscrew them (see pics below).
The motor housing on this treadmill appeared to be the home of some mice. Notice the debris and droppings in one of the pics below.