If this machine has functioned well in the past and this problem has showed up suddenly, with no damage having been done to the machine, then start with the cord. Check for dings, scrapes, anything that could get down into the actual wires inside. If you don't find anything there, then you'll need to take the unit apart where the cord enters and trace it to the motor. You'll need an ohm meter to check continuity. When you find a point where two wires are touching each other or the outside body of the unit that will probably be your problem area.
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If this machine has functioned well in the past and this problem has showed up suddenly, with no damage having been done to the machine, then start with the cord. Check for dings, scrapes, anything that could get down into the actual wires inside. If you don't find anything there, then you'll need to take the unit apart where the cord enters and trace it to the motor. You'll need an ohm meter to check continuity. When you find a point where two wires are touching each other or the outside body of the unit that will probably be your problem area.