I have the Proform Power 990 and my belt continues to loosen and slip to the right. It goes so far that it rubs on the bottom of the platform. No amount of tightening fixes this. I've followed all the directions, tightening the right bolt and loosening the left. This keeps it tightened for a half-mile then it's loose again. Please help.

You have not properly aligned and / or adjusted the tension. If it is SLIDING then you do not have enough Rear Tension. Once you get the right amount of REAR TENSION, then you have to adjust the left right (balance the belt). Your belt should go straight, right now you have it too far to the FRAYED SIDE. Turn the Machine on, have your Hex Wrench in the Bolt and turn it slowly CW and as the belt come back to Center, watch the adjustment. If it continues to go past, then loosen it a little until you have the right balance. Remember REAR TENSION FIRST... THEN BALANCE for Alignment

If the treadmill belt continues to slip even after you have adjusted the rear roller bolts, then it is time to replace the running belt. These belts do stretch over time and must be replaced when they stretch to the point that it can no longer be tightened enough to not slip.
https://www.sportsmith.com/walking-belt/product/31332

what do you mean off like completly off or to one side. you need to balance the tension on the w/belt otherwize the blet will creep to the left or right. if the belt goes to the right tighten the right and loosen the left, when the belt get to where you want it. tighten the side you made the belt move to this will keep it centered

hi you need to tighten the left side a turn or two and loosen the right side one or two. this is while the tread is on 3-4.once it moves over to where you want it. tighten the rite side . PS to tighten or loosen the bolts to the w/belt. clockwize is tight, and counter is loose

The belt does not need to be replaced more than likely. The problem is probably because that adjustment bolt is stripped out causing it loosen and keeping it over on that side. ProForm sells star washers that help them stay steady and not slip out. I would call them and ask for a couple of star washers and a new adjustment bolt for that left side.

If the walking belt slips when walked on, first remove the key and UNPLUG THE POWER CORD. Using the allen wrench, turn both rear roller bolts clockwise 1/4 of a turn. When the walking belt is correctly tightened, you should be able to lift each edge of the walking belt 3 to 4 inches off the walking platform. Be careful to keep the walking belt centered. Then, plug in the power cord, insert the key, and walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened.

Unplug unit remove motor shroud then plug unit back in , press start and begin walking on the belt normally and try to stall the belt to make it slip ( planting foot to stop the belt).
Be careful during this test. If the problem is in the drive belt, either the drive belt will stop when stalled or the front roller will stop when stalled. Here is the procedure for adjusting the drive belt.
Find the motor mounts. Many motors in Icon treadmills have a main pivot joint in which a bolt runs the full length of the motor mount. This bolt will require loosening to provide maximum adjustability. Some models have a single jack bolt and others have an auto tensioner. Single jack bolts are the easiest to adjust.
Find the set bolt(s) on the motor. This is typically run through only one side of the motor mount or may be on both sides but it does not run the full length of the mounting. Loosen this bolt(s) but do not remove. Jack bolt models will have flat mounts and may contain up to four bolts.
Look for a bolt that will tension the motor. If it has this bolt, it typically runs in the same direction parallel with the drive and walking belts. If your treadmill is not equipped with this bolt, skip to Step 4. If your treadmill is equipped with this bolt (very few are) the repair will only require one person. Using 2 7/16" wrenches, make a few turns to this bolt, tighten the bolts and test your treadmill for slipping. If belt continues to slip, and if their is too much tension ( you should be able comfortably twist the drive belt 90 degrees ) then replace drive belt. If the motor, drive belt, and front roller continue to turn when the walking belt is stalled, the problem is with the walking belt slipping on the front roller, here the procedure adjusting the walking belt.
If the walking belt slips when walked on, first remove the key and UNPLUG THE POWER CORD. Using the allen wrench, turn both rear roller bolts clockwise, 1/4 of a turn.When the walking belt is correctly tightened, you should be able to lift each side of the walking belt 3 to 4 inches off the walking platform. Be careful to keep the walking belt centered. Plug in the power cord, insert the key and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened
If the drive belt continues to turn but the front roller stalls, you may have a broken front roller pulley. If this is the problem, you will notice it because the plastic will usually make a loud squealing sound on the metal front roller. The front roller assembly will need to be replaced and the drive belt must be replaced also. We suggest replacing the drive belt as well because it is a low cost item and will most likely, have suffered unusual wear from the problem. Another possibility is that the drive belt has worn down. In some cases, belt dressing may help correct the problem. If not, the drive belt must be replaced.

Unplug unit then remove motor shroud then do the following: If the problem is in the drive belt, either the drive belt will stop when stalled or the front roller will stop when stalled. If the motor, drive belt, and front roller continue to turn when the walking belt is stalled, the problem is with the walking belt slipping on the front roller. If the drive belt continues to turn but the front roller stalls, you may have a broken front roller pulley.

1) Adjusting the running belt if the problem: If the walking belt slips when walked on, UNPLUG THE POWER CORD. Using the hex key, turn both idler roller bolts clockwise, 1/4 of a turn. When the walking belt is correctly tightened, you should be able to lift each edge of the walking belt 2 to 3 in. off the walking platform. Be careful to keep the walking belt centered.Then, plug in the power cord, insert the key, and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened.
2) Adjusting Drive belt: Find the motor mounts. Many motors in Icon treadmills have a main pivot joint in which a bolt runs the full length of the motor mount. This bolt will require loosening to provide maximum adjustability. Some models have a single jack bolt and others have an auto tensioner. Single jack bolts are the easiest to adjust.

If the walking belt slips when walked on, first remove the key and UNPLUG THE POWER CORD. Using the hex key, turn both rear roller bolts clockwise, 1/4 of a turn. When the walking belt is correctly tightened, you should be able to lift each edge of the walking belt 2 to 3 inches above the walking platform. Be careful to keep the walking belt centered. Then, plug in the power cord, insert the key, and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened. Now if this does not solve this problem then check the drive belt and adjust accordingly, also reverse the process of the above mentioned

Loosen the belt incrementally and see if rough or growling sound goes away if so the belt was too tight! Now after loosening the belt and it start slipping then it time to replace belt. If the sound continue then loosen the belt all the way suggest you count the number of turns and right it down so when you need to tighten the belt you won't guess at it. Now that the belt is fully loose with a good light inspect upper and lower areas of the rollers deck and motor compartment.