Check the flag pin. If the motor is overloaded, the flag pin will be low. Also, it is low when the L6470 is first powered on. Therefore, when testing you should confirm that flag goes high, and then stays high during operation. Thermal Warning and Thermal Shutdown both trigger the flag. The only way to clear the flag pin is to call GetStatus.
Note: The Flag pin is ACTIVE Low. So is the Busy pin.
At 12V, 1.3 ohms would produce 9 amps. This could be a problem.