I have a system error pop-up on an HMI of one of my machines. Since it only had a number, I tried to reach the manufacturer for the error code description. They said that this error usually is the result of a bad analog input (in my case, one of the thermocouples). I have regular J-types, which are connected directly to PLC. Here's the thing: they all read exactly what you'd expect, but the manufacturer's techs tell me that it could read OK and still be bad, which I don't understand.
Suppose "+" wire's insulation is damaged and there's a short to the ground, then there would be no reading. And if "-" wire is shorted to the shield, and, let's say, it picks up some EMI, but then, it would either get filtered inside the PLC or it would effect the reading.
So, my question is: How can a thermocouple give a good reading and still be bad?