Thermocouple troubleshooting
The quick way to test this is to simply disconnect the suspect thermocouple and substitute a similar thermocouple. You don't have to install the substitute in the machine. Simply see if the error goes away.
If you do not have a similar thermocouple, make one by using a piece of thermocouple wire. Quick and simple.
You could also swap the wires of the suspect thermocouple with another within the machine. Then see if the error moves also.