Run your treadmill at your speed with no one on it for 40 minutes If it doesn't slow down you have a worn belt and deck or a belt that needs lube.
I have a video explaining this if you need it.
Maintaining Your Home Treadmill Revised at,
http://www.mindbites.com/person/214-Mark
If lubing doesn't help you may have other issues or a worn belt and deck.
E-mail me if you need to
Mark Peterson
March 2008
Automatic Answer
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howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gym/fitness equipment.