After correctly wiring and powering on the instrument according to the wiring diagram, the instrument will first display the thermocouple type, then the instrument's measurement range. The lower row of the digital display shows the set temperature, and the upper row shows the measured temperature. If the upper row of the digital display does not show the temperature of the heating element, but instead displays "OVER," "0000," or "000," it indicates a fault in the instrument's input section. The following tests should be performed:
1. Disconnect the thermocouple from the instrument's thermocouple input terminal, and then short-circuit the instrument's thermocouple input terminal with any wire. When powered on, if the upper row of the digital display shows a value approximately equal to room temperature, it indicates an open circuit in the thermocouple's internal wiring; the thermocouple should be replaced with one of the same type. If the same condition persists, it indicates that the instrument's input terminal was damaged during transportation, and the instrument needs to be replaced.
2. Remove the thermocouple from the faulty instrument and connect the thermocouple from a nearby, normally operating instrument of the same type. After powering on, if the upper row of the original faulty instrument's digital display shows the heating element temperature, it indicates an open circuit in the thermocouple wiring; replace the thermocouple with one of the same type.
3. Remove the faulty thermocouple from the instrument. Use a multimeter set to the resistance (R) *1 range to measure the resistance across the two ends of the thermocouple. If the multimeter shows a very high resistance value, it indicates an open circuit in the thermocouple's internal connection; replace the thermocouple with one of the same type. Otherwise, if there is a certain resistance value, it indicates a problem with the instrument's input terminal, and the instrument should be replaced.
4. If the wiring is correct, but after powering on, the upper row of the instrument's digital display shows negative values, it indicates that the "+" and "-" terminals of the thermocouple connected to the instrument are reversed. Simply reverse the connections.
5. If the wiring is correct and the instrument is operating, but the temperature displayed on the upper row of the digital display differs from the actual measured temperature by 40 to 70 degrees, or even more, it indicates that the instrument's thermocouple type setting is incorrect. Based on the temperature-to-millivolt (mV) correspondence of thermocouples with different calibration types (B, S, K, E, etc.), at the same temperature, the millivolt value produced by a type B thermocouple is the smallest, followed by type S, then type K, and type E produces the largest millivolt value. This principle is used for identification.
By methodically following these diagnostic steps-starting with the simplest non-destructive tests-you can quickly isolate whether the malfunction stems from the thermocouple (open circuit, polarity error, wrong type) or the instrument itself (damaged input). Always document findings, use appropriate personal protective equipment, and consult the specific instrument manual for model-unique details or error code interpretations. This structured approach minimizes downtime and prevents unnecessary component replacements.
