There is a disconnection of the RTD probe from the control panel or there is a bad connection point.
How do we fix it - Check the connection by disconnecting the wires on the back of the controller and restarting the grill. In a pinch, the wires for the RTD probe can be cut down and stripped so the bare wire can be connected to the back of the controller. This will reset the connection and allow the probe to communicate with the controller again. If neither works, replace the controller.
This means that the RTD probe has failed.
How do we fix it- The RTD will need to be replaced.