How To Troubleshoot a 4-20 mA Loop

I worked in technical support for a long time. One of the most common issues my customers had was their 4-20 mA current loop not working as it was supposed to.
In this post, I will explain how to troubleshoot a 4-20 mA loop, which can save you time and effort in hiring someone to fix it for you all and calling technical support.

Tools you need to troubleshoot a 4-20 mA loop

Essentially, you will need two tools to troubleshoot the 4-20 mA current loop: a digital multimeter and a loop simulator/calibrator.

A digital multimeter

A digital voltmeter is an essential tool in analyzing a loop. Using a tool to ensure voltage levels and measuring the current through the loop is the fastest way to verify operation and isolate problems.
how to troubleshoot a 4-20 mA loop

A loop simulator

Another useful tool is a loop simulator/calibrator, which allows the user to set the loop current to known levels and individually power up and test the functionality of the loop devices.
The functions of an advanced loop simulator allow technicians to troubleshoot on the spot without disconnecting wires or “breaking the loop.” Multifunction process simulators can also be used to test 4-20mA loops as well as digital controls.
loop simulator

Main components of a 4-20 mA loop.

The basic layout of every 4-20 mA loop consists of a power supply, a transmitter, and passive loop devices.

The Power source

The supply produces the energy to run the loop.

The Transmitter

The transmitter controls the current through the loop.

The loop device

Loop devices provide feedback to the world, whether it is an indicator displaying the measured quantity from the transmitter or some type of relay output control device.

How To Troubleshoot a 4-20 mA Loop

Many things can cause a malfunctioning 4-20 mA loop. The first step is to determine where the problem is. Problems can be caused by power, wiring, or loop device issues.

The Power Supply

The first step in troubleshooting any circuit is to check the power supplies. Measure the loop power supply voltage, and ensure it is at the proper level.

1) If the supply output is zero, determine if the supply is being powered, if a fuse is blown, or if the supply is damaged.

2) If the supply voltage is a little low, check to see if the supply is unregulated. Variation of the output voltage with load is normal for an unregulated supply.

3) If the supply is regulated and the output is low, it may be caused by a high loop load. Disconnect the loop and measure the voltage output. If the voltage output remains faulty, the supply might be bad.

If the voltage output level returns to the specification, a high loop load might be the cause.

Insert a milliamp meter between the supply and the disconnected wire to measure the loop current.

A loop current much larger than 26mA indicates an excess load on the loop. Possible causes of excessive loop currents are miswiring a ground loop and a problem with the transmitter.

Ensure that the transmitter is installed with the proper polarity. The power supply could be faulty if the current is less than 22mA and the supply output voltage is low.

If the power supply is bad, you have two options: you can repair it. Usually, it could be a bad fuse or burned resistance, and if you can not repair it, please consider replacing the power supply.


Types of 4-20 current loop

One common theme when it comes to 4-20 mA current loop troubleshooting is identifying the type of 4-20 mA. Depending on the transmitter, the 4-20 mA loop can be wired using two wires, three wires sink or source, and four wires isolated.

I have seen that most people confuse this type of 4-20 mA. They try to wire a 3-wire transmitter to a controller that can only take two wires, and this can cause issues with your 4-20 loop.

Types Of 4-20 mA Current Loop

Wrong Wiring

The power supply + terminal should be run to the + terminal of the first item in the loop. The – terminal of the first item on the loop should be run to the + terminal of the second item until the wiring returns to the – terminal of the power supply.

With the loop supply powered, measure the voltages across the loop devices. The voltages on the loop devices should agree with the specifications, and the voltage polarity must agree with the + and—of the terminal block.

If the voltages across all the loop devices are zero and the loop supply is within specification, then there is a break in the loop.

If most, if not all, of the voltage occurs across any one of the loop devices, then there is a problem with that device.

Loop Devices

Loop devices ( usually a transmitter and a controller ) can fail due to improper scaling, incorrect wiring, or electronic failure.


Problems with transmitters usually occur because of wiring problems. The loop and the sensor must be wired properly. Transmitters can be damaged by attaching power to the wrong terminals.

If the transmitter is off, check the power voltage. If the power voltage level is good ( usually 24VDC), check if the device is properly wired.

If the device is wired properly, measure the voltage across the transmitter to check the polarity and ensure that it has sufficient voltage to operate by comparing the measured voltage to the minimum in the specifications for that transmitter. Sensors must be properly attached to the transmitter.

If the transmitter has a display, it will usually show some code when there is a problem, which you can use to determine what is wrong with the sensor or transmitter.

If the transmitter does not have a display, it might have an mA output to indicate its failure.

Usually, this value is between 1 mA and 3.99 mA; you can use the digital multimeter to measure the output current.

How To Measure A 4-20mA Loop Signal Using A Multimeter

Improper loop calibration

Sometimes, the transmitter or a controller displays values that do not represent the variable’s real reading; this might be because the loop needs to be calibrated.

An improperly scaled display will react to the 4-20mA signal, moving up and down predictably but not displaying the proper values for the loop.

Correcting this requires identifying the range of the 4/20mA transmitter and scaling the display to these values. The fact that the indicator is powered introduces another fault condition. 

The controller

If you connect your 4-20 mA from the transmitter to the controller, you might also need to troubleshoot the controller.

The controller issues might be caused by bad wiring, electronics failure, or an uncalibrated loop.

If the controller is not powering up, the first step is to check the power and ensure it is within the range specified by the manufacturer.

If the controller reading is not the same as the sensor reading, this could be that you need to calibrate the 4-20 mA loop.

After loop calibration, if the controller keeps displaying wrong values, disconnect the transmitter from the controller and use the loop simulator to simulate values to the controller.

If it keeps reading different values from the simulator, then the issue is with the controller. If the values are the same, then the issue is with the transmitter.

After figuring out which loop device is the issue, please follow the manufacturer’s instructions to troubleshoot that specific device.

FAQ: How To Troubleshoot a 4-20 mA Loop

How to quickly troubleshoot a 4-20 mA loop?

The quickest way to troubleshoot a loop is to identify what it is doing instead of what it is not doing.
For instance, if you can say that the loop current is within the specification, then you can say that the power supply and transmitter are probably fine.

What can I do if the loop works fine but does not operate properly when the level goes up?

If the loop operates fine until it reaches a certain level, it doesn’t operate properly. This can indicate that there are too many devices in the loop.

How to troubleshoot a 4-20mA signal?

The first step is to test the power supply. We must measure the loop power supply and ensure that it has the proper power level(18-36VDC).
If the power supply is reading as zero, check if the power source is powered, a fuse is blown, or the power supply is damaged.

Key Takeaways

The easiest way to troubleshoot the 4-20 mA loop is to divide it into three sections: the power, the wiring, and the loop devices.

Check the power to see if it meets the requirements. Are the devices on? If not, check the power.

If yes, check the wiring. Is the loop wired properly? If yes, then check the loop devices.

This way your 4-20 mA current loop troubleshooting should be easy and effective.

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