Sensors are a critical component of the thermal loop. They provide a millivolt signal to the controller which, in turn, applies an advanced algorithm and cycles power to the heater. But what happens if the reporting of this temperature is flawed because the sensor is inappropriately isolated from the process? The sensors and controls never ask why a system needs more heat. They only know that it does. A misreading could cause a constant and inappropriate increase of heat, which can lead directly to reduced heater life, premature failure and unexpected downtime of customer processes and throughput.
That being said, the simple need for maintenance can easily be detected with the use of thermocouples and datalogging to create a baseline temperature or profile for a heater.