If your treadmill belt keeps sliding to the left (or right) you may have a bigger problem than the rear roller being off of alignment. Look on the inside of the endcap (especially Nordic Track and ProForm) and look for cracks or a broken end cap all together. That’s the main problem.
There is a fix that can save you $40 or more (some end caps are one piece connecting both sides and can run @ $90). The manufacturer thinks plastic can hold the pressure of the belt tension over time. This is just not feasible. If you want to try to fix this yourself, here’s what you can do.
Go to your hardware store and buy an oversized washer large enough to cover the hole in the endcap that the bolt fits through. A 1″ or 1.5″ washer with a 1/4″ hole should work. If you’re not sure on the size of the hole, bring the roller bolt with you. You may need to buy a longer bolt to make it easier to reach the roller when you get home to put the bolt back in. Most of the bolts for Nordic Track and ProForm are 1/4″ #20 in size. An extra 1/2″ to inch should be plenty. Since the head of the bolt will be outside of the endcap, it doesn’t need to be an allen or socket head bolt, just don’t get a bolt that would require a screwdriver. Hex head would be best and make adjustment easier with a wrench.
So, you put the bolt through washer and then place it back into the endcap. The washer will then be on the outside of endcap (this is why you may want a longer bolt since it is now displaced farther from the roller). I haven’t had any problems with this not holding up after doing this fix.
Saves you money and gets your treadmill back to work. You just need to adjust the tension and alignment.