Why might a 4‑20 mA sensor not read correctly on a PLC?

Why Might a 4-20 mA Sensor Not Read Correctly on a PLC?

Introduction

The 4-20 mA signal has been the backbone of industrial automation for decades. It is simple, reliable, and resistant to electrical noise, making it the most trusted way to send analog data between sensors and PLCs.

When a PLC fails to read a 4-20 mA sensor correctly, entire processes can be disrupted. Incorrect values may trigger false alarms, reduce product quality, or even stop production.

This guide explains why 4-20 mA signals fail, how to troubleshoot step by step, and which tools keep automation systems running reliably.

What Is a 4-20 mA Signal and Why Is It Used in Automation?

A 4-20 mA signal is a current loop used to transmit analog values from sensors to PLC input modules. The signal starts at 4 mA, called the “live zero,” and increases linearly up to 20 mA, which represents the maximum measured value.

The live zero serves two purposes. It powers loop-powered devices and helps detect wiring faults, since a reading below 4 mA indicates a problem. Unlike voltage signals, current signals do not lose accuracy over long distances. This makes the 4-20 mA method reliable for factories and industrial plants where sensors may be located hundreds of meters from controllers.

Automation systems still depend on 4-20 mA because it is simple, cost-effective, and compatible with nearly every PLC available today.

How Does a PLC Handle 4-20 mA Inputs?

When a sensor transmits a 4-20 mA signal, the PLC’s analog input module converts it into a digital value using an Analog-to-Digital Converter (ADC). For example, in a module scaled for 0–100 PSI, 4 mA represents 0 PSI and 20 mA represents 100 PSI.

The PLC stores this input as counts in engineering units. Engineers then scale the counts into usable data such as temperature, pressure, or flow. If scaling is incorrect, the PLC may read 12 mA as 80 PSI when it should read 50 PSI. Such errors can lead to unsafe conditions and costly mistakes.

Understanding the 4-20 mA Loop and Its Core Components

A 4-20 mA loop is a simple series circuit that connects sensors to PLCs for reliable data transfer. Each device plays a specific role in ensuring accurate signal transmission.

Transmitter (sensor): Produces the 4-20 mA output signal carrying measurement data.

Power supply: Provides 24 VDC in most industrial systems to run the loop.

Wiring: Carries the loop current between devices, often through shielded cables.

PLC analog input module: Receives the current and converts it into digital values for control.

All devices in the loop are wired in series. If one device fails or is connected incorrectly, the entire signal path is interrupted.

Why Might a 4-20 mA Sensor Not Read Correctly on a PLC?

Incorrect readings usually come from three areas: wiring issues, input scaling, and noise problems. By checking each one, engineers can restore accurate PLC inputs quickly.

Sensor Wiring Issues
Wiring faults are one of the most common causes of PLC misreadings. Loose terminals, reversed polarity, or broken conductors can drop the loop current below 4 mA, which shows as a failed sensor. Long cable runs may also cause voltage drops if the wire gauge is too small.

Shared commons create further errors. When multiple transmitters use the same return path, readings may interfere across channels. Following wiring diagrams and using shielded twisted pair cables prevents these problems.

Input Scaling and Calibration Errors
A mismatch between transmitter range and PLC configuration leads to wrong values. For example, a transmitter scaled for 0–300 PSI will not read correctly on a PLC input set for 0–150 PSI.

Calibration must also be exact. Even small deviations, such as a sensor outputting 3.9 mA at live zero, can produce inaccurate results. Proper scaling and calibration ensure that current values match engineering units.

Noise Problems and Ground Loops
Electrical noise from motors, drives, or power lines can distort a 4-20 mA signal. Ground loops occur when devices are grounded at different points, creating stray currents that interfere with readings.

Using shielded cables, correct grounding, and isolators such as opto-couplers or transformer-based isolators reduces these issues.

Step-by-Step Troubleshooting of a 4-20 mA Signal in a PLC

Troubleshooting a 4-20 mA signal in a PLC works best when done in a clear order. Each step rules out one possible cause until the fault is found.

Check the power supply. Confirm that 24 VDC is present and stable before testing other components.

Measure loop current. Place a multimeter in series with the transmitter and verify the signal is within the 4–20 mA range.

Compare transmitter and PLC readings. If the transmitter output is correct but the PLC shows wrong values, the fault lies in the input module or its configuration.

Simulate the transmitter. Use a loop calibrator or current source to test if the PLC input reads correctly without the actual sensor.

Check wiring and grounding. Inspect for loose terminals, broken conductors, poor shielding, or ground faults.

Following this sequence ensures each part of the loop is tested. This approach makes it faster to locate and correct the exact cause of incorrect PLC readings.

Tools Needed for Testing a 4-20 mA Loop

Having the correct tools makes troubleshooting a 4-20 mA loop faster and more accurate.

Multimeter with mA capability – measures loop current directly to confirm sensor output.

Clamp-on current meter – checks current flow without breaking the loop.

Loop calibrator or signal simulator – generates known current values to test PLC inputs.

250Ω resistor – converts loop current to 1–5 VDC, allowing easy voltage-based checks.

With these tools, you can verify signal accuracy, detect wiring issues, and diagnose faults with confidence.

How to Configure PLC Input Modules for 4-20 mA Signals

Configuring PLC input modules correctly ensures accurate interpretation of 4-20 mA sensor signals.

Set the input type to current mode. Choose “current” on the PLC card so it accepts 4-20 mA signals.

Define the scaling range. Match the PLC scaling to the transmitter’s output (for example, 0–100 PSI).

Enable calibration checks. Verify readings with a multimeter to confirm accuracy.

Match the wiring configuration. Ensure connections follow the module’s sourcing or sinking setup.

Proper configuration allows the PLC to interpret the full signal range from the sensor without errors.

Grounding, Shielding, and Isolation in 4-20 mA Loops

Ground loops and interference can distort PLC readings. Following best practices keeps signals accurate and stable.

Use shielded twisted pair wiring. It reduces electromagnetic noise and improves signal quality.

Ground shields at one end only. This prevents circulating currents that create false readings.

Employ signal isolators. Opto-isolators or transformer isolators protect loops when multiple signals share wiring.

Verify isolation ratings. Check PLC analog input modules to ensure proper separation between circuits.

These practices reduce noise, improve signal reliability, and maintain accurate PLC readings in every loop.

Scaling and Calibration in Detail

Scaling ensures the PLC input matches real-world measurement values. For example:

4 mA = 0 PSI

20 mA = 200 PSI

Calibration corrects small deviations in sensor output. A sensor reading 4.01 mA instead of 4.00 mA may create a false low value. Using calibration software or manual checks with a multimeter keeps readings accurate.

Real-World Troubleshooting Example of a PLC 4-20 mA Loop

A food processing plant reported erratic temperature readings. The transmitter was outputting the correct 12 mA for 50°C, but the PLC displayed 70°C.

The cause was incorrect scaling. The PLC input card had been set for 0–150°C while the transmitter was configured for 0–100°C.

Correcting the scaling restored accurate readings. This shows how a simple mismatch between transmitter range and PLC configuration can create major errors.

Comparison Chart: Troubleshooting Methods & Tools

Method
Pros
Cons
Best Use Case

Multimeter in series
Accurate, low cost
Requires breaking the loop
Quick verification

Clamp-on current meter
Non-intrusive, easy to use
Less precise
Routine checks

Loop calibrator/simulator
Highly accurate, flexible
Expensive
Commissioning, calibration

PLC online diagnostics
Real-time data logging
Depends on PLC features
Advanced troubleshooting

Why Forums and Collaboration Matter in 4-20 mA Troubleshooting

Engineers often face troubleshooting challenges that vary from case to case. Control engineering forums and online communities provide direct peer support.

Sharing technical details helps. Posting wiring diagrams, loop currents, or PLC configurations often reveals faults faster than working alone.

Collaboration spreads best practices. Lessons shared across industries, from food processing to oil and gas automation, help improve reliability in new applications.

Forums and collaboration save time, reduce errors, and give engineers access to solutions tested in real-world systems.

Related Comparison Context

4-20 mA signals are vital for sensor feedback, but the same principle of correct configuration applies in motor control.

Servo vs Stepper Motors. Servos use closed-loop feedback for precision. Steppers are simpler but can lose steps without feedback.

BLDC Motor Comparison. Brushless DC motors provide higher efficiency and lower maintenance than brushed motors.

Motor Selection in Automation. The right choice depends on torque, accuracy, and feedback method—similar to how correct input scaling ensures accurate PLC readings.

In both sensor loops and motor systems, proper configuration is the foundation of reliable automation performance.

Conclusion

A 4-20 mA sensor may fail to read correctly on a PLC due to wiring errors, scaling mismatches, or electrical noise. A structured troubleshooting process and the right tools help engineers find and fix faults quickly.

Regular calibration and preventive maintenance reduce the chance of errors in critical loops. Consistent checks of wiring, grounding, and configuration ensure long-term reliability of PLC systems.

Key Takeaway

Always check wiring and polarity first.

Match scaling ranges between the transmitter and the PLC input.

Use shielded cables and ground them properly to prevent electrical noise.

Test signals with a multimeter, loop calibrator, or simulator before replacing hardware.

FAQs About 4-20 mA PLC Troubleshooting

How do you troubleshoot a 4-20 mA signal?
Check the loop power and confirm that the supply voltage is stable. Measure the current loop with a multimeter, then compare the value with the PLC input. Make sure the scaling in the controller matches the transmitter range.

How to connect a 4-20 mA sensor to a PLC?
Wire the transmitter in series with the power supply and the PLC analog input module. Confirm wiring polarity is correct so that the 4 mA live zero and 20 mA full-scale output give accurate readings.

How to test a 4-20 mA signal?
Use a loop calibrator to generate a known current. Another method is inserting a 250Ω resistor in series and measuring voltage (1–5 VDC) with a multimeter. Both methods confirm if the PLC is receiving an accurate signal.

What is a 4-20 mA fault?
A reading below 4 mA usually means an open loop, broken wiring, or a failed transmitter. Erratic signals often result from poor grounding or missing shielding.

What is the likelihood of a ground loop developing in a working system?
Ground loops occur when multiple PLCs or input modules have different earth connections. Using isolated analog inputs and grounding shields at one end reduces the chance of failure.

Related reading: Interfacing sensors and actuators with PLCs · What is Modbus? · Diagnose common servo drive faults · Design & troubleshoot PLC motion control