A thermocouple does not measure the temperature itself but the difference to its reference junction. The device has to know that junction’s temperature; it is called the reference or compensation temperature.

Internal

Measured at the protective earth of the mould connector. On devices from hardware revision 501 a sensor is fitted as standard for this. This is the usual choice.

External

You fit the measuring point yourself, where the thermocouple wires change to copper wires. This saves the compensating cable in the extension of the mould cable and makes sense when the transition point is far from the device.

If the selected source does not deliver a usable value, the device switches to a substitute source by itself and reports “Reference temperature source faulty”. It therefore keeps controlling, though possibly less accurately. As soon as the selected source delivers good values again, it switches back without intervention. If the message persists, check the external measuring point and its wiring.