Troubleshooting guide

Why Is a 4-20 mA Loop Reading Wrong or Stuck?

Open circuits, ground loops, insufficient supply voltage, wrong range and failed transmitters: a step-by-step way to isolate a faulty 4-20 mA loop.

Controls & Instrumentation · Last reviewed October 15, 2026

A 4-20 mA loop is a series circuit: power supply, transmitter, wiring and input resistor (or analog input), all carrying the same current. That makes it easy to troubleshoot, because you can measure the current anywhere in the loop and compare it with the reading.

Follow the site procedures when breaking a loop or working in an area that needs permits. Opening a loop on a running process can trip a control or safety action, so notify operations first.

Reading at the controller	Likely cause	What to check
0 mA or below 3.6 mA	Open circuit: broken wire, loose terminal, blown fuse, or a dead transmitter	Continuity of the loop, terminals, fuse and supply voltage at the transmitter
Above 21 mA, or full scale	Short or ground fault in the wiring, a failed transmitter, or a sensor out of range	Insulation to ground, wiring, sensor and the transmitter fault indication
Stuck at a fixed value	A failed transmitter output stage, a held output, or a controller input fixed in simulation	Measure the loop current with a milliammeter. Check for forced values in the controller
Reading wrong, current looks correct	Wrong range or scaling in the controller	Compare the controller scaling with the transmitter range
Reading wrong, current is not what the process implies	Transmitter out of calibration, a wrong sensor reading, or a clogged impulse line	Calibration check at zero and span, and process connection condition
Noisy or jumping reading	Electrical noise, poor shield grounding, ground loop, or a loose terminal	Shield grounded at one end only, separation from power cables, terminal tightness
Works at low readings, fails at high	Not enough loop supply voltage at 20 mA	Supply voltage against the transmitter minimum plus the voltage drops in the loop

The supply-voltage check

At 20 mA the loop must still leave the transmitter its minimum operating voltage (often 10 to 12 V). Example: a 24 V supply, a 250 ohm input resistor, and 150 ohm of wiring. The drop at 20 mA is 20 mA × (250 + 150) ohm = 8 V. The transmitter then has 24 − 8 = 16 V, which is enough for a 12 V minimum. Add a second 250 ohm indicator in series and the drop becomes 13 V, leaving 11 V, which is below 12 V: the transmitter cannot reach 20 mA.

A good order of attack

Put a milliammeter in series (or measure across the input resistor and convert with Ohm's law).
Compare the current with the expected value for the known process condition.
If the current is right and the reading is wrong, the fault is in the scaling or input channel.
If the current is wrong, simulate the signal at the transmitter terminals to separate field wiring from the transmitter.
Record the as-found and as-left values.

A second example: reading the loop with a meter

A voltmeter across the 250 ohm input resistor reads 2.4 V.
Loop current = 2.4 V ÷ 250 ohm = 9.6 mA.
On a 0 to 10 bar transmitter, 9.6 mA corresponds to (9.6 − 4) ÷ 16 × 10 = 3.5 bar.

If the controller shows a different value from 3.5 bar while the loop carries 9.6 mA, the fault is in the controller scaling or its input channel, not in the field wiring or the transmitter.

Preventive checks

Calibrate transmitters at zero, mid-range and span on the interval your procedure sets, and record as-found and as-left values.
Check terminal tightness and look for corrosion in junction boxes.
Keep signal cables away from power cables, and ground the shield at one end only.
Test the loop alarm limits so that a broken wire is reported.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

More troubleshooting

Questions

What is a HART signal?

A digital signal superimposed on the 4-20 mA current. It lets a handheld communicator read and configure the transmitter without disturbing the analog reading.

Should the shield be grounded at both ends?

For low-frequency instrument signals, grounding at one end only is the usual practice, to avoid ground loops. Follow the site standard.

What is a loop calibrator?

A handheld instrument that can supply or measure a 4-20 mA signal. It lets a technician simulate a transmitter or read the loop current without breaking the circuit in some models.

Why does an open loop read 0 mA?

No current can flow through a broken circuit. Because the live zero is 4 mA, the controller can tell a true zero reading from a broken wire.

How do I find a ground loop?

Disconnect the shield and the ground connections one at a time and watch whether the noise changes. A ground loop usually comes from the signal being grounded at more than one point.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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