I get a FAIL message on my digital controller. According to the instructions, this means the thermocouple is no good. But it looks fine--not broken or corroded.
It is possible that the thermocouple was struck and damaged. Here is a simple test, which works with most brands of controllers:
1) Unplug. Remove the controller board.
2) Remove the 2 thermocouple wires from their connections on the back of the board.
3) Cut a paperclip in half. Insert a u-shaped paperclip piece, or other piece of wire, between the thermocouple connectors.
4) Plug in the kiln. If the board reads room temperature, replace the thermocouple. If you have inserted the paperclip at the thermocouple leads on the back of the controller, and you still get a FAIL message, the controller may be defective.