There is a procedure for adjusting the treadmill belt. Almost all the treadmill follows the same procedure. If you wish to know how to adjust the treadmill belt properly then check the below added link. Even after that if you face the same problem then you have replace the running belt of your treadmill.
http://electro-medical.blogspot.com/
If you wish to get some details about treadmill calibration and error code troubleshooting then check the following link.
http://electro-medical.blogspot.com/
Can be the lower limit switch be faulty. Of you wish to get some details about treadmill repair and service then check the following link
http://electro-medical.blogspot.com/
the resistance is done by a magnet and a motor that moves it closer or farther away from the flywheel. possible the motor is bad or not adjusted properly