4–20 mA Loop Reads 0 mA, 3.6 mA or 22 mA: What It Means

A healthy 4–20 mA loop should normally stay somewhere between its lower and upper measuring limits. When you see exactly 0 mA, roughly 3.6 mA or around 22 mA, the number is usually telling you more than “the measurement is wrong.”

These values often represent three different fault categories:

• 0 mA: the current loop is probably open, unpowered or measured incorrectly
• 3.6 mA: the transmitter may be deliberately reporting a low-side fault
• 22 mA: the transmitter may be deliberately reporting a high-side fault

There is one important warning, though: these values are common conventions, not universal laws. Always check the manual and configuration of the exact transmitter.

How a Normal 4–20 mA Loop Works

A transmitter converts a process value—such as pressure, temperature, flow or level—into current.

For a pressure transmitter scaled from 0 to 10 bar:

• 4 mA represents 0 bar
• 12 mA represents 5 bar
• 20 mA represents 10 bar

The advantage of starting at 4 mA rather than 0 mA is that the system can distinguish a genuine zero process measurement from certain electrical failures.

A properly operating transmitter may also produce values slightly below 4 mA or above 20 mA when the process moves just outside its calibrated range. Those saturation values should not immediately be confused with transmitter-failure currents.

What Does 0 mA Mean?

A reading of approximately 0 mA usually means that no useful current is flowing through the loop.

Common causes include:

• No 24V DC loop power
• A blown fuse
• A broken field cable
• A disconnected terminal
• Incorrect transmitter polarity
• An open analog-input circuit
• A failed transmitter
• The multimeter connected incorrectly
• An isolator or barrier without power

In a two-wire transmitter loop, the same pair of conductors normally supplies the transmitter and carries the measurement current. If the circuit opens anywhere, current stops throughout the complete series loop.

A reading near 0 mA is generally not a standard NAMUR NE43 low-fault signal. Devices using NAMUR-style failure reporting commonly drive the output to 3.6 mA or below for a low alarm rather than deliberately switching to exactly zero. Siemens transmitters, for example, can be configured to indicate sensor failure at no more than 3.6 mA or at least 21 mA.

Start by Checking Loop Power

Measure the DC voltage directly at the power-supply output.

Then measure the voltage at the transmitter terminals while the loop is connected.

If 24V is available at the supply but not at the field transmitter, follow the circuit through:

• Fuses
• Terminal blocks
• Signal isolators
• Intrinsic-safety barriers
• Junction boxes
• Disconnect plugs
• Cable joints

A completely open circuit may allow you to measure almost the full supply voltage across the break. That voltage does not prove that current can flow.

Check the Ammeter Connection

To measure current, a multimeter must be inserted in series with the loop.

Do not place a meter set to current mode directly across a 24V supply. Current mode has very low internal resistance, so connecting it in parallel can short the supply and blow the meter fuse.

Also confirm that:

• The red lead is in the correct current socket
• The meter fuse is intact
• DC current mode is selected
• The loop is reconnected through the meter

A blown internal meter fuse can make a healthy loop appear completely dead after the meter is inserted.

What Does 3.6 mA Mean?

A stable reading around 3.6 mA often means that the transmitter is powered and communicating its own internal fault condition.

This is commonly called:

• Fail low
• Downscale alarm
• Low fault current
• Burnout low
• Sensor-failure low

NAMUR NE43-style equipment typically uses a low failure signal of 3.6 mA or less. Pepperl+Fuchs equipment, for example, specifies a downscale fault at no more than 3.6 mA, while Siemens instruments allow a similar selectable low failure response.

Possible reasons include:

• Broken sensor element
• Shorted temperature sensor
• Failed pressure cell
• Internal transmitter electronics fault
• Invalid calibration
• Measurement outside the permitted sensor range
• Incorrect sensor type
• Failed remote probe connection
• Device startup or diagnostic mode

The exact cause is usually stored in the transmitter’s diagnostic menu or available through HART communication.

3.6 mA Is Not Always a Broken Cable

This distinction is important.

If you measure 3.6 mA, current is still flowing. The loop is therefore unlikely to be completely open.

The transmitter may be intentionally pulling the output down to tell the PLC that it has detected a problem.

Check:

1. The transmitter display
2. Active diagnostic codes
3. HART status
4. Sensor wiring
5. Configured failure direction
6. Lower-range and saturation settings

Do not scale 3.6 mA as a valid process value. Your PLC should identify it as a bad signal and generate an appropriate instrument alarm.

What Does 22 mA Mean?

A reading around 22 mA often represents a transmitter fault reported in the high direction.

This may be described as:

• Fail high
• Upscale alarm
• High fault current
• Burnout high
• Sensor-failure high

Siemens SITRANS devices can be configured to output approximately 20.5 mA or 22 mA during a failure, while other Siemens equipment specifies a general fault-current threshold of 21 mA or higher.

The reason for using 22 mA instead of exactly 20 mA is straightforward: 20 mA is already a valid full-scale process value. A higher current allows the control system to distinguish a genuine maximum measurement from a reported device failure.

Possible causes include:

• Internal transmitter failure
• Sensor open circuit
• Sensor short circuit
• Measurement beyond the configured range
• Invalid device configuration
• Electronics temperature fault
• Failed pressure or temperature sensing element
• A configured high-direction burnout response

The transmitter’s failure direction may be selectable. One plant may configure every instrument to fail low, while another may choose fail high according to its control and safety philosophy.

Do Not Confuse Saturation With Failure

Many smart transmitters allow a small amount of current beyond the normal 4–20 mA range.

A device might use values such as:

• Approximately 3.8 mA for low saturation
• Approximately 20.5 mA for high saturation
• No more than 3.6 mA for a low fault
• At least 21 mA or a configured 22 mA for a high fault

Emerson documentation, for example, separates process saturation values from transmitter alarm values so that the receiving system can distinguish an out-of-range measurement from a device failure.

Therefore:

• 3.9 mA may represent a slightly underrange but still valid process condition
• 3.6 mA is more likely to represent a fault
• 20.5 mA may represent process overrange
• 22 mA is more likely to represent a high failure alarm

Exact limits vary. Use the transmitter manual rather than hard-coding one set of numbers across every instrument.

Step-by-Step Testing Procedure

1. Read the Current at the PLC

Check the PLC raw value and the scaled engineering value.

Confirm that the analog input is configured for 4–20 mA rather than:

• 0–20 mA
• 0–10V
• ±10V
• A disabled channel

Also examine the module diagnostics for wire break, overflow and underflow.

2. Measure the Actual Loop Current

Insert a suitable meter or loop calibrator in series.

Compare the measured current with the PLC reading.

• If both show the same abnormal value, continue toward the transmitter.
• If the meter shows 12 mA but the PLC reports 0 mA, investigate the analog input, common wiring and channel configuration.

3. Measure Voltage at the Transmitter

A loop-powered transmitter requires enough voltage to operate while driving current through the total loop resistance.

The required voltage is consumed by:

• The transmitter
• Cable resistance
• Analog-input resistance
• Isolators
• Barriers
• Displays and recorders

If the available voltage is too low, the transmitter may fail to reach the requested current—especially near 20 mA.

4. Disconnect the Field Signal and Use a Calibrator

Inject known current values into the PLC input:

• 4 mA
• 12 mA
• 20 mA

If the PLC reads these values correctly, its analog input and scaling are probably healthy.

Next, simulate the process sensor or measure the transmitter output separately. This divides the system into manageable sections instead of replacing equipment blindly.

5. Check Device Diagnostics

For a powered smart transmitter producing 3.6 or 22 mA, connect using:

• The local display
• HART communicator
• Manufacturer software
• Device web interface, where available

Read the actual fault code. “Fail high” only tells you how the device reports the fault; it does not identify the failed component.

Recommended PLC Alarm Logic

Do not treat every current between 0 and 24 mA as a valid process measurement.

A practical structure may use separate states:

• Below approximately 3.6 mA: instrument or loop fault
• 3.6 to 3.8 mA: diagnostic or uncertain low region
• Normal measurement range: valid process value
• 20 to approximately 20.5 mA: possible high saturation
• Above approximately 21 mA: high instrument fault

Add a short time delay where needed so electrical noise does not create repeated alarms.

The exact limits should match the transmitter specification, analog-input module and site standard.

Quick Interpretation Table

These are useful starting points, not replacements for the device manual.

Final Thoughts

A 4–20 mA loop carries both the measurement and, in many smart instruments, diagnostic information.

Zero current usually points toward a dead or open circuit. A stable 3.6 mA often means the powered transmitter has deliberately reported a low fault. A reading around 22 mA usually indicates a configured high failure alarm.

Measure the real current, check the voltage available at the transmitter and then read the device diagnostics.

Most importantly, do not scale fault currents into believable process values. A failed level transmitter showing 3.6 mA should create an instrument alarm—not quietly tell the operator that the tank is almost empty.