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That sounds like a matter of the speed sensor not working. However, in most cases if the speed sensor is not providing any signal indicating that the treadmill is running, the treadmill shuts down with a speed sensor error. That hints to me, that either this treadmill is engineered and designed with unusually bad reactions to a rather common failure - unlikely! - or that something else, more expensive and harder to fix, is wrong with the treadmill.
Were you thinking of fixing it yourself? in a case like this, it might be best not to, to state my opinion frankly.
If you wish to have the treadmill serviced by a local NordicTrack tech repair firm, let me know your region (by city/state or zip code) and I will give you contact information for local repair firms specializing in NordicTrack repairs.
In this particular model... actually, what is your particular model? the 3500, the 3600, or something else?