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.
Safety first
Wrong analogue readings can drive a process to act incorrectly. Treat readings as suspect until verified. Isolate before working on wiring.
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
Wiring fault (open / short on the loop)
Most likelyAn open 4-20mA loop reads low/zero; a short or miswire can pin it. Broken screen/return affects 0-10V too.
- 2
Scaling / range mismatch in the controller
#2The controller's scaling doesn't match the sensor's range, so the value is wrong even with a healthy signal.
- 3
Sensor out of calibration / drifted
#3The transmitter has drifted or needs recalibration.
- 4
Power/loop supply problem
#4A loop without its supply (or wrong loop powering) gives no or wrong signal.
- 5
Failed sensor / input channel
Least likelyThe transmitter or the analogue input channel has failed.
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 ModeMeasure the actual signal (mA or V) at the sensor and at the input, and compare to the reading.
A sensible signal that corresponds to the real process value.
Signal is right but the value is wrong — it's a scaling problem.
Signal wrong (zero/pinned) — check wiring, loop supply, then the sensor.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Analogue reading wrong
→ step 2 - 2decision
Does the measured signal match the real process value?
Yes→ step 3No→ step 4 - 3decision
Does the controller scaling match the sensor range?
Yes→ step 5No→ step 6 - 4decision
Does a known injected signal read correctly at the input?
Yes→ step 7No→ step 8 - 5result
Signal and scaling fine — check/redo calibration.
- 6result
Scaling mismatch — correct the range/units in the controller.
- 7result
Input fine — sensor/transmitter or wiring at fault; calibrate/replace.
- 8result
Input channel fault — controller maintenance.
Common mistakes apprentices make
- Trusting the displayed value without measuring the actual signal.
- Mismatched scaling between sensor range and controller config.
- Forgetting loop power on a 4-20mA transmitter.
- Replacing the sensor when it just needs recalibration or rescaling.
When to stop & escalate
Calibration against a reference should follow your procedure/standards. A failed analogue input channel means controller maintenance per site procedure (with backups).
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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