Do not walk on the treadmill while checking and adjusting the belt tracking.
Put the key in and start the treadmill. Set the speed to the slowest setting and let it run for a minute or so.
If the belt is not tracking in the center, remove the key and unplug the power cord.
Use the Allen wrench supplied with the treadmill and adjust the rear rollers as needed to center the walking belt.
If the belt is tracking to the left of center, turn the left roller adjustment screw clockwise 1/2 of a turn.
If the belt is tracking to the right of center, turn the left roller adjustment screw counter clock-wise 1/2 of a turn.
After adjustment, restore power and recheck the tracking.
Repeat until belt is tracking in the center.