A controller cannot correctly control a process if it is unable measure the process variable. In most cases the system can detect a loss input signal, and within a few seconds it displays an error message such “Open” or “Input Fail” and takes the appropriate actions such activating alarms or turning outputs off.
Some models have fixed actions under signal fail conditions, but it is often possible to choose its behavior as break high (act as if the process had gone high) or break low (act as if the process had gone low, or even going to preset output values.