Common treadmill problems include speed changes, slipping belts, overheated belts, no display and a burning odor. The owner should fix the problem as soon as possible, before it causes more problems.
If the treadmill's speed seems to change during use, you should check the owner's manual for a recommended speed setting, and set the treadmill to that speed without getting on. Watch the belt for a couple minutes for any hitches. If the speed changes, the treadmill most likely needs a belt replacement.
A slipping feeling while using the treadmill usually indicates an issue with the belt's alignment or tightness. To fix this, align the belt and tighten it until you can lift it two to three inches in its center.
If the belt overheats, the treadmill may automatically shut off for safety. This overheating results from friction. To fix it, lubricate the belt. If that doesn't work, it is probably necessary to replace the belt.
A missing display is typically a minor issue. Check the power cord to make sure it's still plugged in, and check the treadmill's cords for any damage.
If the treadmill has a burning odor, turn if off and unplug it immediately to avoid a fire. Possible causes include friction between the deck and back or an electrical short.