A) Inspect for any obstruction that could cause belt to stop. Reset power to the unit at the On/Off switch.
B) Inspect cable and connection of opto (speed) sensor. Repair and replace damaged cable or connection of the opto(speed) sensor.
C) Inspect alignment of the opto (speed) sensor. Sensor should be centered evenly over its target. Adjust or replace opto (speed) sensor if necessary.