Belt does not move

That sounds like the motor is running and the belt is slipping. Can you hear the motor turning? Try adjusting the belt tension.

https://www.google.com/search?q=NordicTrack+C1650

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https://www.google.com/search?q=nordictrack+c1650+treadmill+troubleshooting

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Either you will replace the belt or have to adjust the motor/belt tension.
1. The motor can be loosened by a couple of bolts. This allows the motor to be moved to adjust the tension of the belt.
2. The tension can be adjusted properly by turning the motor on with the foot pedal; low speed.
3. While the motor is running move the motor in a fashion that will loosen or tighten the belt. The correct location will allow the motor to run at it's highest speed.
4. Re-tighten the bolts and retest the machine.

check belt

Remove side cover to check...a little confused about which make/ model you have from the above detail
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To get the belt to the center you will see two adjustment bolts on the very back of the machine that require an allen wrench to adjust. Whichever side the belt is going towards is the side you will need to tighten. The best way to do it is to turn it a half of a turn then try and run the machine. if that does not help then loosen the other side a half turn, continuing until it pulls that belt back to the middle.

If it is a belt problem, you should be able to hear the motor running. If you cant hear the motor running then it has to be the foot control or cord or possibly the motor.

if you hear the motor moving with no belt turning the problem is either the front roller or the drive belt. If there is no motor movement then this is usually an indication of a bad control board located under the lower section of the treadmill.

99% probability that the drive belt has come off. Can you hear the motor whizzing when it should be turning the drum, but the drum doesn't move? If so then it will certainly be the belt off.... If you can't hear the motor then the motor its self may be shot. Most machines allow you to remove the back. If the belt IS off, then fit it to the MOTOR first, then slip it over the edge of the large wheel on the back of the drum and turn the wheel by hand to ease the belt back onto the wheel.

Now the question: Why did it come off? Your happiest hope is that you had a very heavy and uneven load in when it came off and that the load shook the machine so violently that it caused the belt to come off. This is good, don't overload the machine and it won't happen again. If this isn't the case, and it happens again, hope the belt is worn out and stretched (Hate to say it, but this is quite unlikely. Take it to the place where you can buy a new belt, measure your belt against a new one to find out). LAstly, sadly most likely and most expensive is the possibility that the belt came off because the rear bearing in the drum is worn out and allowed the drum to run out of line so the belt fell off. To test for this, open the front loading door of the machine and slip your fingers onto the drum while your thumb rests on the outer casing of the machine. Lift and squeeze the drum with your fingers, pulling it towards your thumb. The drum should rock back and forth on its springs, this is normal. If you hear a little clunk and you can see the inner drum moving while the outer drum (where the rubber gasket is fixed to the drum) stays stationary in relation to the inner drum moving up and down, you probably have a shot rear bearing. For most machines, this fault is not an ameture fix and its time to call in the repair man. Be prepared for the news that (more than 50% of the time) this problem is more expensive to fix that the machine is worth if it is getting on in years. If this is the case, weigh up the cost of the repair to an old out of warranty machine, against putting the cash toward a new machine that will give you the peace of mind of a warranty if thisgs go wrong

Hi,
By removing the top cover, or the turning wheel cover,
you will be able to see the belt.
If the belt is not broken, then try to adjust the belt tension,
by adjusting either the height of the motor, or the tension wheel.

I HAVE THE SAME PROBLEM. I HEAR THE MOTOR AND BELT IS MOVING BUT NEEDLE IS NOT MOVING