On a sprint 2 there are no thread break sensors on the upper portion of the machine. The only thread break sensor is under the bobbin plate, The red arrow shows the sensor that should flash on and off as the round metal arm moves back and forth when the machine is operating. Chance are this is just dirty and needs to be cleaned. The blue arrow shows the pivot point that needs to be lubricated just a little. Remove the bobbin plate, remove the small arm, and clean and lub just a little. replace and you should be good to go. Can't get the picture to upload will try again.