Replacement Temperature Sensor/ Proof of Fire Switch/ Snap Disc 125 Degrees (52C) for Castle pellet stoves.
This is a thermodisc, or more commonly known as a snap switch. It is what relays to the controller that a fire started in the burn pot and the ignitor can now shut down. If you find that your stove is shutting down shortly after the pellets have begun burning, it may indicate that this switch is faulty.