Often when I come out and diagnose that a part needs to be replaced, then another service call is typically required to install the part or parts. We may be able to save at least one service call if someone in your home is a just a little handy.
1.) We would need to take off the motor cover. Then without reaching inside due to exposed electronics.
2.) You would need to set the speed to about 2 miles per hour while standing on the side rails. Then with one foot, stomp down on the walk belt to make it stop.
3.) If everything stops including the motor, then there is no problem.
4.) If the front roller keeps turning but the belt stops, this means that either the belt is too loose, or is worn out and needs to be replaced. Belts typically wear on the the inside faster than the outside. When new they have a heavy fabric, burlap type of textured surface. This grips the drive roller very well when the belt is tightened properly, even though it is lubricated. When the underside of the belt is worn, it is too smooth to grip the front drive roller.
5.) If the front roller stops when you stomp down on the walk belt, but the pulley on the roller still turns even partially, then typically the front roller needs to be replaced.
6.) If the walk belt and the front roller and roller pulley stop, but the drive belt still turns, even partially, then the drive belt either needs to be tightened (if there is an adjustment) or it needs to be replaced.
7.) Once you know what part or parts need to be replaced send me that info along with the model number of your treadmill. This is not the model number on the top like NordicTrack A2300, but its a long number of the manufacturers label on the bottom of the treadmill frame. This can be located on the frame on the front below the motor cover, or on fold up treadmills it could be on the bottom of the frame that is exposed when the deck is lifted for storage.
Here are some examples of manufacturer tags and model numbers: