A slight slippage could be felt which typically is due to the walk belt being too loose, or the drive belt being too loose due to wear.
The tension on the walk belt felt fine, so I removed the motor cover to check the drive belt. I knew that we had found the culprit. The drive belt was loose and was slipping on the motor pulley under the weight of the user. I showed the customer how that at low speed I could hold the walk belt in place with my foot and the motor pulley just turned inside of the stopped drive belt.
The solution was to replace the motor drive belt with a new belt. To do that you need to:
1.) Unplug the treadmill.
2.) Loosen the rear belt adjustment bolts evenly, about 10 counter-clock-wise turns each using the appropriate size allen wrench or the one that came with your treadmill. Just like you would do if you were going to lube your belt.
3.) Remove the phillips screws that hold on the motor cover and remove the motor cover.
4.) There is a bolt on the right side of the front roller that holds the front roller in the proper position. Mark the position of the axle location on the frame with a marker so that you can reposition it in the same place upon reassembly.
5.) Remove the nut first and then remove the bolt that is threaded through the front roller axle.
6.) You can now lift up the right side of the front roller out of its position, then pull to the right side of the treadmill until the left side of the axle comes out of the hole enough so that you can remove the drive belt off over the end of the axle as well as the motor pulley.
7.) Slip the new belt onto the motor pulley as well as the front roller pulley and slide the roller axle back into the hole and drop the right side of the front roller back into place.
8.) While holding the end of the roller in proper position aligning it with the mark that you made on the frame, thread the bolt back through the hole in the frame and through the threaded axle. Once it passes through the axle you need to put the locking nut back on. Once the bolt is seated and the roller is where it originally was located you can tighten the lock nut against the axle.
9.) Replace the motor cover and screws.
10.) Thread the rear roller adjustment bolts back in 10 turns clockwise. Now plug the treadmill in, turn on the treadmill and start it at a low speed. Make any minor adjustments needed to the rear roller adjustment bolts to center the walk belt.