A crack in the thermocouple’s welded tip can cause the temperature display to bounce wildly or show FAIL. Loose thermocouple wires can cause the same problem.
Suppose you replace the thermocouple, and the display still shows FAIL. Here is a simple thermocouple test that you can perform yourself. It works with most brands of controllers and will help you determine if a display problem is due to the thermocouple or the controller:
1) Unplug or disconnect the kiln.
2) Remove the controller board from the switch box. On Paragon kilns, remove the four corner screws from the controller faceplate. Lift the faceplate out of the switch box.
3) Look at the back of the controller circuit board. You should see two thermocouple wires connected to the bottom of the board. Disconnect those two wires. (The older controllers have screw connectors; the Sentry versions have lever connectors or push-button connectors.)
4) Insert a thin piece of wire such as a bent paperclip in the thermocouple connectors.
5) Place the controller faceplate onto the switchbox with a couple of screws.
6) Plug in the kiln/connect the power. If the board reads room temperature, replace the thermocouple. If it reads FAIL, you will need to have the controller checked.