A slipping treadmill belt is caused by too much friction between the walking belt and the running deck. The belt may need lubrication or it could be too loose. The treadmill belt will slip when the belt is not tightened to specifications.
Before making any adjustments to the treadmill, be sure to unplug the treadmill from the power source. First, make sure the belt has been properly lubricated within the past 60 miles of use. Always lubricate with silicone lubricant designed for treadmills. Next, use a screw driver to uncover the motor and examine for fragments of walking belt. If there are noticeable fragments, it is likely time to purchase a new walking belt.
Once the belt is checked and determined to be in usable condition, check the tension of the belt. Most treadmill belts will lift 2 to 3 inches off of the center of the running deck when the belt is properly tightened. If it lifts more or less than 2 to 3 inches, the belt needs to be tightened.
To tighten the walking belt you will need an Allen wrench to adjust the rear roller bolts on the back end of the treadmill. Turn the roller bolts clockwise to tighten the belt and counterclockwise to loosen it. Be sure to turn each bolt one quarter of a turn in the desired direction, then test the tension by pulling up the belt in the center of the running deck.
Once the walking belt is able to be lifted to the desired 2 to 3 inches, test the belt by plugging in the machine and begin walking on it. Increase the speed slowly until it is determined the slipping has stopped or repeat steps if slipping continues to occur.