A thermocouple fault may create an obvious alarm or a plausible but incorrect value. Wrong alloy extension wire, reversed polarity, a loose junction, electrical noise, oxidation, and poor attachment can shift the reading or make it respond slowly.
The indicated temperature should be compared with an independent reference and with the expected response to a controlled heat change. Sensor type, controller configuration, polarity, continuity, insulation, cold-junction compensation, and wiring transitions should be verified. Intermittent faults often appear when cables move or the machine reaches operating temperature.
Sensors and extension wiring must be replaced with the specified type and installed to represent the intended surface or medium. Control tuning should not be changed until the measurement path is proven. A stable but biased thermocouple can drive a repeatable process at the wrong actual temperature.