Troubleshooting Notes
This switch should be "NC" or normally closed or "ON". If the switch detects 200°F or more the switch rolls out to "OPEN" and the power is cut telling the controller to stop feeding and the room blower will run non-stop. Pressing the red button on the switch will "RESET" the switch to the NC position provided the switch's metal face is <200°F on the metal face.
High limit switches do not usually fail without some kind of event, like a power surge. Generally if the switch fails the reset button will no longer hold the switch in the normally closed position. If this has happened, it would be a good idea to remove the controller and inspect the electrical circuitry on both sides of the board for signs of a power arc or controller board component failure. IF replacing the switch does not remedy the problem, you may need a new controller.