I just repaired such a treadmill, with report of motor not working. All that was amiss was that the motor brushes were warn out, replaced in 10 min job.
  In this case the motor would stop /start if the belt was moved.
  1 DISCONNECT mains plug
  2 Check you have DISCONNECTED mains plug.
  3 Unscrew the small screws (Prob, 6) of motor cover.
  4 Slide off cover.
  5 Unscrew black plastic motor brush plugs(2)
  6 Pop out old brushes, insert new ones.
  7 Replace plugs,cover and screws.
                
 Ken J H
                
 October 2009