The thermocouple is a temperature sensor that protrudes into the firing chamber. The wires that connect the thermocouple to the digital controller are color-coded. It is important to follow the color coding when installing a new thermocouple.
If you reverse the thermocouple wires, you will get an error message. It varies depending on the brand of controller. For the Sentry 3-key and 12-key controllers, "TCR" means reversed thermocouple wires.
If the thermocouple wires are reversed at one location, reversing the wires at another location will not fully correct the problem.
Example: You remove the controller and find that the thermocouple wires are on the correct terminals on the back of the controller. The wires are reversed at the thermocouple block. If you reverse the wires at the controller, the temperature readout will appear to be correct, but it will not be accurate.
The thermocouple wires must be on their correct terminals at both the thermocouple block and the controller. Reversing wires at the controller to avoid gaining access to the thermocouple block will not fully correct the problem.