Temperature Sensor Fault — Thermocouple Break

Temperature controller or PLC reports thermocouple open circuit (upscale burnout). Temperature reading is invalid — may show maximum range or error code.

Probable Causes (ranked)

• Thermocouple wire broken — fatigue from heat cycling or mechanical damage
• Thermocouple terminal connection loose at controller or junction box
• Thermocouple sheath damaged — element exposed to process and corroded through
• Wrong thermocouple type connected — Type J in Type K input gives wrong reading then error
• Extension wire splice failed — corrosion or loose crimp at junction

Fix Steps

1. 1 Check thermocouple connections at the controller terminal strip — tighten screws
2. 2 Measure resistance across thermocouple leads at controller — open circuit confirms break
3. 3 Inspect thermocouple probe at process insertion point — look for sheath damage or corrosion
4. 4 Verify thermocouple type matches controller configuration — J, K, T, etc.
5. 5 If thermocouple is broken, replace with correct type and length
6. 6 Check extension wire connections at any intermediate junction boxes — re-crimp or replace
7. 7 After replacement, verify reading against a known reference or second temperature instrument

Suggested Replacement Parts

Parts matched to fault category: Thermocouple

Prices are estimates. Verify part numbers match your equipment before ordering.

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