My son (8) got a s670 SkyViper stunt drone for Christmas. Now, two days later, when we connect the battery wires, one propeller immediately goes full throttle, even though the controller isn't even switched on. Once we turn the controller on, we can throttle up the rest of the propellers, but the one stays one full sped the whole time until the battery is unplugged.

We've tried disconnecting the battery and charging it fully, that didn't work.

I have no idea how to fix this.