QualifiedMedium risk

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.

Start fault-finding

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.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
Wiring fault (open / short on the loop)
Most likely
An 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
#2
The 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
#3
The transmitter has drifted or needs recalibration.
4
Power/loop supply problem
#4
A loop without its supply (or wrong loop powering) gives no or wrong signal.
5
Failed sensor / input channel
Least likely
The transmitter or the analogue input channel has failed.

Reports are saved on this device to reflect what you actually find.

Premium fault tree

The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 3

Measure the actual signal (mA or V) at the sensor and at the input, and compare to the reading.

Expected reading

A sensible signal that corresponds to the real process value.

Your actual reading

If it passes

Signal is right but the value is wrong — it's a scaling problem.

If it fails

Signal wrong (zero/pinned) — check wiring, loop supply, then the sensor.

Often leads toSensor intermittent / drops out randomly Encoder feedback fault (position/speed wrong)

View all expected readings at once

1. Measure 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.

2. If the signal is good, check the controller's scaling/range against the sensor's range.

Scaling matches the sensor's range and units.

3. If the signal is wrong, check loop wiring/supply, then inject a known signal to test the input channel.

A known injected signal reads correctly at the input.

Full test sequence

The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Fault-finding flowchart

The same logic as a decision tree.

1
start
Analogue reading wrong
→ step 2
2
decision
Does the measured signal match the real process value?
Yes→ step 3No→ step 4
3
decision
Does the controller scaling match the sensor range?
Yes→ step 5No→ step 6
4
decision
Does a known injected signal read correctly at the input?
Yes→ step 7No→ step 8
5
result
Signal and scaling fine — check/redo calibration.
6
result
Scaling mismatch — correct the range/units in the controller.
7
result
Input fine — sensor/transmitter or wiring at fault; calibrate/replace.
8
result
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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5 causes3 test steps

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