My treadmill is a Proform 520X, like 6-8 years old, a lower end model.
So over the last month or so, the belt would occasionally slip. Within the last week it quit completely. I mean it will run with no one on it, but forget it when I go on it. Trying to figure its problem. The drive roller is always spinning, but not driving the walking belt, so I ruled out motor & drive issues. Still got adjustment left on the belt too. If I over-tighten the belt, it will start to spin, but still not good. The deck surface is like a mylar film, that you are supposedly not to lube. But I'm thinking of giving that a try with lube lightly at least as it doesn't seem slippery. I think friction of the belt to the deck is keeping it from going, and that the belt is still OK. So that is the cheapest fix too, like $15. Start there, if that isn't it, go with the belt next, about $100. What is any one else's thoughts and experiences?