4-20 mA Transmitter Has Power but PLC Shows No Reading Malaysia
4-20 mA Transmitter Has Power but PLC Shows No Reading Malaysia
The transmitter display is on.
The sensor appears to be operating.
But the PLC shows zero, no reading or an invalid process value.
Does this mean the PLC analog input is faulty?
Not necessarily.
A powered transmitter does not automatically mean that the complete 4-20 mA current loop is working correctly.
The problem may involve the transmitter output, loop wiring, polarity, PLC terminal, analog input configuration, loop power arrangement or PLC scaling.
For suitable troubleshooting applications, the FNIRSI SG-003A Signal Generator can provide known test signals to help determine whether the PLC receiving side responds correctly.
For maintenance and instrumentation teams in Malaysia, this can help separate a field-side problem from a PLC-side problem.
Transmitter Power and Signal Are Two Different Things
One of the first mistakes during troubleshooting is assuming:
Transmitter display is ON = 4-20 mA signal is OK
This is not always true.
The transmitter may have power while the signal path to the PLC is interrupted or incorrectly connected.
Think of the system as separate stages:
Sensor > Transmitter > 4-20 mA Loop > PLC Analog Input > PLC Program > HMI
The transmitter being powered only confirms part of this chain.
First Confirm the Expected Signal
Before testing anything, identify what signal the transmitter should provide.
For example:
Pressure Transmitter
Range:
0-10 bar
Output:
4-20 mA
The expected relationship is:
4 mA = 0 bar
8 mA = 2.5 bar
12 mA = 5 bar
16 mA = 7.5 bar
20 mA = 10 bar
If the PLC shows no reading, determine where this expected signal disappears.
Problem 1: Incorrect Loop Wiring
A transmitter can be powered while the signal wiring is incorrect.
Check the manufacturer's wiring diagram carefully.
Look for:
Wrong terminals
Loose connections
Incorrect polarity
Disconnected signal wire
Incorrect common connection
Wrong PLC terminal
Do not rely only on wire colours.
Trace the actual circuit.
Problem 2: Wrong PLC Analog Input Channel
The transmitter may be connected to one PLC input while the program monitors another.
For example:
Physical wiring:
AI05
PLC software:
AI03
The transmitter can be operating correctly, but the HMI may still show no meaningful reading because the software is looking at the wrong channel.
Confirm:
Field Wire > Terminal > PLC Channel > PLC Tag > HMI Tag
Problem 3: PLC Input Is Configured for the Wrong Signal
Some PLC analog input modules support several input types.
Examples may include:
4-20 mA
0-20 mA
0-10 V
If a 4-20 mA transmitter is connected but the PLC input is configured incorrectly, the reading may not behave as expected.
Check both:
Hardware connection
and
Software configuration
The correct setup depends on the PLC model.
Problem 4: Loop Power Arrangement Is Incorrect
Not every 4-20 mA system is wired in exactly the same way.
Different transmitters and PLC modules may have different power and signal arrangements.
Before changing wiring, determine:
How is the transmitter powered?
Is the loop externally powered?
What does the PLC input require?
Which terminals are current input and common?
Is the test instrument sourcing or simulating the required signal?
Refer to the actual equipment manuals.
Incorrect assumptions about loop power can result in a system that does not work even though individual devices appear powered.
Problem 5: Transmitter Has Power but No Valid Output
The transmitter itself may be powered but unable to provide the expected process signal.
Possible causes depend on the instrument.
Examples may include:
Sensor fault
Transmitter configuration problem
Internal fault
Incorrect output setting
Sensor connection problem
Process measurement problem
Check the transmitter diagnostics where available.
Problem 6: PLC Raw Input Is Present but HMI Shows Nothing
The problem may occur after the analog input.
Use the PLC programming software, where possible, to check the raw input value.
If the raw value changes but the HMI still shows:
0
or
No Reading
investigate:
PLC scaling
HMI tag
Communication
Data type
Engineering-unit conversion
Display configuration
This is not necessarily an analog input hardware problem.
How Can a Known Signal Help?
Suppose you disconnect or isolate the field side according to the approved test procedure and correctly apply a known test signal to the PLC receiving side.
The PLC is supposed to represent:
4-20 mA = 0-100%
You can then observe its response at suitable test points.
For example:
4 mA = 0%
8 mA = 25%
12 mA = 50%
16 mA = 75%
20 mA = 100%
This gives the technician a controlled input instead of relying on an uncertain field transmitter.
If the PLC Responds Correctly
Suppose the original transmitter gives no usable PLC reading.
But with a correctly applied known signal:
4 mA displays 0%
12 mA displays 50%
20 mA displays 100%
The PLC receiving side is responding correctly under the test conditions.
The investigation can then move towards:
Field transmitter
Field wiring
Loop power
Junction boxes
Field terminals
Transmitter configuration
This can prevent unnecessary replacement of the PLC module.
If the PLC Still Shows No Reading
Suppose a known signal is correctly applied but the PLC still shows no response.
Investigate:
Correct PLC terminals
Correct analog channel
Current input configuration
Input module status
PLC software configuration
Raw analog value
Module diagnostics
Possible analog input hardware fault
A PLC hardware fault becomes more plausible only after simpler causes have been eliminated.
Example: Pressure Transmitter Is ON but PLC Shows 0 Bar
A pressure transmitter is powered and its local display shows:
6 bar
The transmitter range is:
0-10 bar = 4-20 mA
At 6 bar, a linear transmitter would normally produce approximately:
13.6 mA
But the PLC displays:
0 bar
There are now two important questions:
Is the expected current actually reaching the PLC?
and
Does the PLC respond correctly when given a known current signal?
These two questions help divide the problem into field side and receiving side.
Example: Level Transmitter Has Display but HMI Shows No Level
A tank level transmitter has a working local display.
The tank is approximately half full.
The system is configured as:
4-20 mA = 0-100%
At approximately 50%, the expected signal is around:
12 mA
If the HMI still shows zero, check whether the PLC actually receives the expected current.
If a known 12 mA test signal produces approximately 50%, investigate the transmitter loop and field wiring.
If 12 mA still produces no PLC response, investigate the PLC side.
Example: New Transmitter Installed but PLC Still Shows Zero
This happens frequently during replacement work.
A maintenance technician replaces an old transmitter.
The new transmitter powers up correctly.
But the PLC still shows zero.
Before concluding that the new transmitter is defective, compare:
Old wiring diagram
New transmitter wiring diagram
Output signal type
Power requirements
Terminal functions
PLC input configuration
The replacement transmitter may not have exactly the same wiring arrangement as the old instrument.
Check Junction Boxes and Intermediate Terminals
Industrial signals often do not travel directly from transmitter to PLC.
The path may be:
Transmitter > Junction Box > Marshalling Panel > Control Panel > PLC
Every intermediate connection creates another possible troubleshooting point.
A loose terminal inside a junction box can interrupt the signal even though the transmitter itself has power.
Trace the complete loop.
Check for Wiring Changes After Maintenance
If the system worked before maintenance but stopped afterwards, ask:
What changed?
Was a transmitter replaced?
Was the control panel rewired?
Was a junction box opened?
Was a PLC module changed?
Was the PLC program modified?
Was a cable disconnected?
Recent changes are often valuable clues.
Do Not Change PLC Hardware First
A PLC analog input module may be one of the more expensive components in the loop.
Before replacing it, verify:
Transmitter output
Wiring continuity
Terminal connections
Signal type
Loop power
PLC configuration
Raw PLC input
Response to a known test signal
This provides much stronger evidence before hardware replacement.
FNIRSI SG-003A for Troubleshooting
For suitable applications, the FNIRSI SG-003A can help technicians introduce a known signal while troubleshooting industrial control systems.
Potential applications include:
4-20 mA troubleshooting
PLC analog input testing
Transmitter troubleshooting
Loop checking
Control-panel commissioning
Factory maintenance
Industrial automation
Instrumentation work
The key advantage is being able to ask:
Does the receiving system work when the input signal is known?
Signal Testing Is Not Full Transmitter Calibration
If the PLC responds correctly to a known signal, this does not prove that the transmitter accurately measures the physical process.
For example, fully evaluating a pressure transmitter may require a suitable pressure source and reference.
Similarly, temperature calibration may require appropriate temperature reference equipment.
Signal simulation is primarily useful for functional testing, commissioning and troubleshooting unless the test procedure specifically defines otherwise.
Important Safety Checks
Before disconnecting a transmitter or applying a test signal:
Review the wiring diagram
Confirm the signal type
Check the permitted input range
Identify the correct terminals
Confirm polarity
Understand the loop power arrangement
Check the PLC program
Determine what equipment may respond
Follow site isolation and safety procedures
Changing a process signal can potentially operate:
Pumps
Valves
Motors
Alarms
Interlocks
Automatic sequences
Do not inject test signals into an active system without understanding the consequences.
Looking for FNIRSI SG-003A in Malaysia?
MTM Precision Sdn Bhd supplies the FNIRSI SG-003A Signal Generator in Malaysia for PLC troubleshooting, 4-20 mA loop checking, industrial automation and factory maintenance applications.
If your transmitter has power but the PLC shows no reading, send us a photo of the transmitter nameplate, wiring terminals, PLC analog input module or wiring diagram through WhatsApp.
We can help evaluate whether the SG-003A is suitable for your troubleshooting application.
MTM Precision Sdn Bhd
Showroom & Service Centre
No 29-1 & 2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my
Serving customers across Selangor, Kuala Lumpur, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak.
04 Oct 2026
