Take the end caps and motor shroud off visually inspect the frame tension bolts to see if anything is bent. then loosen, ( count how rotation it took to loosen the so when tighten the belt up you won't have to guest at the tension ) the belt and edges of the belt to see if the weave is uneven this cause the belt to travel. If you can tip the treadmill on its side to inspect underneath to see if anything skewed. Take your time and observe the mechanical operation you may have the above a 1/2 a dozen time before you see the problem, other than that anything short of replacing the belt this my recommendation. Good Luck to you
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Take the end caps and motor shroud off visually inspect the frame tension bolts to see if anything is bent. then loosen, ( count how rotation it took to loosen the so when tighten the belt up you won't have to guest at the tension ) the belt and edges of the belt to see if the weave is uneven this cause the belt to travel. If you can tip the treadmill on its side to inspect underneath to see if anything skewed. Take your time and observe the mechanical operation you may have the above a 1/2 a dozen time before you see the problem, other than that anything short of replacing the belt this my recommendation. Good Luck to you