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You will need to test the windings on the motor with a meter, first of all. Powered Off, unplugged, disconnect one side of each winding on the motor, and measure across to another lead with the meter set to Resistance or Ohms. If the reading is not fairly low (2 to 50 Ohms at most, roughly) then the winding is open... or else you're measuring across the wrong two points perhaps. One of the points should match to the one you have disconnected with that little resistance.
Then you will need to observe the lights on the motor controller board when you power On the machine (with the covers off, of course.) To see if they indicate an error condition.
And with the meter set to voltage, see if there is power getting to the windings of the motor when you have Start-ed the treadmill going.
1: if there is no power to the windings, the power supply or motor controller board may be bad (measure the power supply's voltages, of course, to distinguish.)
2: if there is power to the windings, you probably have an open winding or a shorted winding. And the motor would have to be replaced. (I presume the machine is out of warranty over 1 year old... if it is newer, don't even open it, which will void the warranty, in that case.)
3: if there is nothing detectable wrong with the windings, and the voltages are all present, but the motor won't start, check that any needed safety key is present and conclude the motor controller board is defective.
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