Thermocouple open or reversed (wrong temperature reading)
A thermocouple gives a wrong temperature — reads way off, shows an open/sensor-break, or moves the wrong way — so the heat control can't work properly.
Safety first
A wrong temperature reading can cause runaway heating or no heat. Ensure the over-temperature safeguard is independent and working. Isolate before wiring work.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Open thermocouple / sensor break
Most likelyA broken thermocouple or wiring shows a sensor-break and the controller can't read temperature.
- 2
Reversed polarity
#2Thermocouple leads swapped make the reading move the wrong way or read incorrectly.
- 3
Wrong thermocouple type / extension wire
#3A mismatch between the thermocouple type and the controller setting (or wrong extension wire) gives a wrong reading.
- 4
Poor junction / thermal contact
Least likelyA loose or badly-placed junction reads inaccurately.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeCheck the controller for a sensor-break/open indication and the reading's plausibility.
A plausible reading with no sensor-break.
Reading present — check polarity, type, and placement.
Sensor-break/open — check the thermocouple and wiring continuity.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Thermocouple reading wrong
→ step 2 - 2decision
Is there a plausible reading with no sensor-break?
Yes→ step 3No→ step 4 - 3decision
Are polarity and thermocouple type/wire correct?
Yes→ step 5No→ step 6 - 4result
Sensor-break/open — check thermocouple and wiring continuity.
- 5decision
Is the junction sound with good thermal contact?
Yes→ step 7No→ step 8 - 6result
Reversed polarity or type mismatch — correct it.
- 7result
Re-verify the reading against a reference.
- 8result
Poor junction/placement — remake/relocate it.
Common mistakes apprentices make
- Reversing thermocouple polarity (they're polarity-sensitive).
- Using the wrong extension wire or controller type setting.
- Ignoring a sensor-break indication.
- Poor junction placement giving a misleading reading.
When to stop & escalate
Calibration/verification against a reference follows your procedure. A heating process relying on a single temperature input should have an independent over-temperature safeguard — confirm it exists.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Related faults
Heater overshooting or oscillating around setpoint
The process temperature overshoots the setpoint then cycles above and below it, instead of settling — poor control that can spoil product or trip high-limits.
Analogue sensor (4-20mA / 0-10V) reading wrong
An analogue sensor (pressure, level, temperature) gives a wrong or fixed reading — stuck at zero, pinned at full scale, or simply not matching reality.
Solid-state relay (SSR) stuck on — heat won't switch off
A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.
Learn the theory
How the gear and circuits behind this fault actually work.