First Let us know what is Thermocouple : A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created.
If the thermocouple is disconnected from PLC, the resistance value will be infinite, due to which PLC will show the maximum counts for the Open circuit.