When I step on the treadmill, the runner stops and

Unplug unit then remove motor shroud the walk on the belt normally and try to stall the belt to make it slip. Be careful during this test. If the problem is in the drive belt, either the drive belt will stop when stalled or the front roller will stop when stalled.
Find the set bolt(s) on the motor. This is typically run through only one side of the motor mount or may be on both sides but it does not run the full length of the mounting..
Look for a bolt that will tension the motor. If it has this bolt, it typically runs in the same direction parallel with the drive and walking belts. If your treadmill is not equipped with a bolt as described above, the drive belt must be tensioned by hand. While one person applied increased tension to the drive belt, the other person should tighten the motor set bolt(s) that are described in Step If your treadmill is equipped with this bolt (very few are) the repair will only require one person.
If the motor, drive belt, and front roller continue to turn when the walking belt is stalled, the problem is with the walking belt slipping on the front roller.
If the walking belt slips when walked on. Using the allen wrench, turn both rear roller bolts clockwise, 1/4 of a turn. When the walking belt is correctly tightened, you should be able to lift each side of the walking belt 3 to 4 inches off the walking platform. Be careful to keep the walking belt centered. Plug in the power cord, insert the key and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened.