Troubleshooting the PLC system
Troubleshooting the PLC system (on photo: Mitsubishi PLC type MELSEC; credit:

PLC troubleshooting methods

PLC troubleshooting can be performed in many different ways and as the engineer’s experience always plays the major role in successful resolving of such issues, five methods have been developed. Let’s describe these five most common methods for troubleshooting the PLC systems:

  1. Troubleshooting ground loops
  2. Diagnostic PLC indicators
  3. Troubleshooting PLC inputs
  4. Troubleshooting PLC outputs
  5. Troubleshooting the CPU

At the end of this article check the summary of above described PLC troubleshooting methods.

1. Troubleshooting ground loops

It is a good idea to keep a stock of replacement parts on hand. This practice will minimize downtime resulting from component failure. In a failure situation, having the right spare in stock can mean a shutdown of only minutes, instead of hours or days.

As a rule of thumb, the amount of a spare part stocked should be 10% of the number of that part used. If a part is used infrequently, then less than 10% of that particular part can be stocked.

Main CPU board components should have one spare each, regardless of how many CPUs are being used. Each power supply, whether main or auxiliary, should also have a backup. Certain applications may require a complete CPU rack as a standby spare.

This extreme case exists when a downed system must be brought into operation immediately, leaving no time to determine which CPU board has failed.

If a module must be replaced, the user should make sure that the replacement module being installed is the correct type. Some I/O systems allow modules to be replaced while power is still applied, but others may require that power be removed. If replacing a module solves the problem, but the failure reoccurs in a relatively short period, the user should check the inductive loads.

The inductive loads may be generating voltage and current spikes, in which case, external suppression may be necessary. If the module’s fuse blows again after it is replaced, the problem may be that the module’s output current limit is being exceeded or that the output device is shorted.

Ground loop created by shielded cable grounded at both ends
Figure 1 – Ground loop created by shielded cable grounded at both ends

As mentioned above, a ground loop condition occurs when two or more electrical paths exist in a ground line.

For example, in Figure 1, the transducers and transmitter are connected to ground at the chassis (or device enclosure) and connected to an analog input card through a shielded cable. The shield connects to both chassis grounds, thereby creating a path for current to flow from one ground to another since both grounds have different potentials. The current flowing through the shield could be as high as several amperes, which would induce significant magnetic fields in the signal transmission.

This could create interference that would result in a possible misreading of the analog signal! To avoid this problem, the shield should be connected to ground on only one side of the chassis, preferably the PLC side.

In the example shown in Figure 1, the shield should only be connected to ground at the analog input interface.

To check for a ground loop, disconnect the ground wire at the ground termination and measure the resistance from the wire to the termination point where it is connected (see Figure 2). The meter should read a large ohm value. If a low ohm value occurs across this gap, circuit continuity exists, meaning that the system has at least one ground loop.

Procedure for identifying ground loops
Figure 2 – Procedure for identifying ground loops

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2. Diagnostic PLC indicators

LED status indicators can provide much information about field devices, wiring, and I/O modules. Most input/output modules have at least a single indicator—input modules normally have a power indicator, while output modules normally have a logic indicator.

For an input module, a lit power LED indicates that the input device is activated and that its signal is present at the module. This indicator alone cannot isolate malfunctions to the module, so some manufacturers provide an additional diagnostic indicator, a logic indicator. An ON logic LED indicates that the input signal has been recognized by the logic section of the input circuit.

If the logic and power indicators do not match, then the module is unable to transfer the incoming signal to the processor correctly.

This indicates a module malfunction. An output module’s logic indicator functions similarly to an input module’s logic indicator. When it is ON, the logic LED indicates that the module’s logic circuitry has recognized a command from the processor to turn ON.

In addition to the logic indicator, some output modules incorporate either a blown fuse indicator or a power indicator or both. A blown fuse indicator indicates the status of the protective fuse in the output circuit, while a power indicator shows that power is being applied to the load.

Like the power and logic indicators in an input module, if both LEDs are not ON simultaneously, the output module is malfunctioning.

LED indicators greatly assist the troubleshooting process. With both power and logic indicators, the user can immediately pinpoint a malfunctioning module or circuit. LED indicators, however, cannot diagnose all possible problems; instead, they serve as preliminary signs of system malfunctions.

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3. Troubleshooting PLC inputs

If the field device connected to an input module does not seem to turn ON, a problem may exist somewhere between the L1 connection and the terminal connection to the module. An input module’s status indicators can provide information about the field device, the module, and the field device’s wiring to the module that will help pinpoint the problem.

The first step in diagnosing the problem is to place the PLC in standby mode, so that it is not activating the output. This allows the field device to be manually activated (e.g., a limit switch can be manually closed).

When the field device is activated, the module’s power status indicator should turn ON, indicating that power continuity exists. If the indicator is ON, then wiring is not the cause of the problem.

The next step is to evaluate the PLC’s reading of the input module. This can be accomplished using the PLC’s test mode, which reads the inputs and executes the program but does not activate the outputs. In this mode, the PLC’s display should either show a 1 in the image table bit corresponding to the activated field device or the contact’s reference instruction should become highlighted when the device provides continuity (see Figure 3).

If the PLC is reading the device correctly, then the problem is not located in the input module. If it does not read the device correctly, then the module could be faulty. The logic side of the module may not be operating correctly, or its optical isolator may be blown. Moreover, one of the module’s interfacing channels could be faulty. In this case, the module must be replaced.

Highlighted contact indicating power continuity
Figure 3 – Highlighted contact indicating power continuity

If the module does not read the field device’s signal, then further tests are required. Bad wiring, a faulty field device, a faulty module, or an improper voltage between the field device and the module could be causing the problem.

First, close the field device and measure the voltage to the input module. The meter should display the voltage of the signal (e.g., 120 volts AC). If the proper voltage is present, the input module is faulty because it is not recognizing the signal. If the measured voltage is 10–15% below the proper signal voltage, then the problem lies in the source voltage to the field device.

If no voltage is present, then either the wiring or the field device is the cause of the problem. Check the wiring connection to the module to ensure that the wire is secured at the terminal or terminal blocks.

To further pinpoint the problem, check that voltage is present at the field device. With the device activated, measure the voltage across the device using a voltmeter. If no voltage is present on the load side of the device (the side that connects to the module), then the input device is faulty.

If there is power, then the problem lies in the wiring from the input device to the module. In this case, the wiring must be traced to find the problem.

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4. Troubleshooting PLC outputs

PLC output interfaces also contain status indicators that provide useful troubleshooting information. Like the troubleshooting of PLC inputs, the first step in troubleshooting outputs is to isolate the problem to either the module, the field device, or the wiring.

IMPORTANT! At the output module, ensure that the source power for switching the output is at the correct level. In a 120 VAC system, this value should be within 10% of the rated value (i.e., between 108 and 132 volts AC).

Also, examine the output module to see if it has a blown fuse. If it does have a blown fuse, check the fuse’s rated value. Furthermore, check the output device’s current requirements to determine if the device is pulling too much current.

If the output module receives the command to turn ON from the processor yet the module’s output status does not turn ON accordingly, then the output module is faulty. If the indicator turns ON but the field device does not energize, check for voltage at the output terminal to ensure that the switching device is operational. If no voltage is present, then the module should be replaced.

If voltage is present, then the problem lies in the wiring or the field device. At this point, make sure that the field wiring to the module’s terminal or to the terminal block has a good connection and that no wires are broken. After checking the module, check that the field device is working properly.

Measure the voltage coming to the field device while the output module is ON, making sure that the return line is well connected to the device. If there is power yet the device does not respond, then the field device is faulty.

Another method for checking the field device is to test it without using the output module. Remove the output wiring and connect the field device directly to the power source. If the field device does not respond, then it is faulty. If the field device responds, then the problem lies in the wiring between the device and the output module.

Check the wiring, looking for broken wires along the wire path.

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5. Troubleshooting the CPU

PLCs also provide diagnostic indicators that show the status of the PLC and the CPU. Such indicators include power OK, memory OK, and communications OKconditions. First, check that the PLC is receiving enough power from the transformer to supply all the loads. If the PLC is still not working, check for voltage supply drop in the control circuit or for blown fuses.

If the PLC does not come up even with proper power, then the problem lies in the CPU, and this is very bad.

The diagnostic indicators on the front of the CPU will show a fault in either memory or communications. If one of these indicators is lit, the CPU may need to be replaced.

Summary of PLC troubleshooting methods

In conclusion, the best method for diagnosing input/output malfunctions is to isolate the problem to the module, the field device, or the wiring. If both power and logic indicators are available, then module failures become readily apparent.

The first step in solving the problem is to take a voltage measurement to determine if the proper voltage level is present at the input or output terminal.

If the voltage is adequate at the terminal and the module is not responding, then the module should be replaced. If the replacement module has no effect, then field wiring may be the problem. A proper voltage level at the output terminal while the output device is OFF also indicates an error in the field wiring. If an output rung is activated but the LED indicator is OFF, then the module is faulty.

If a malfunction cannot be traced to the I/O module, then the module connectors should be inspected for poor contact or misalignment.

Finally, check for broken wires under connector terminals and cold solder joints on module terminals.

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Reference // PLC Start Up and Maintenance by Industrial Text & Video Company

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

One Comment

  1. Hatem Khaloua
    Oct 25, 2016

    Hi all
    Great post, very helpful many thx for sharing..
    Plz keep doing your good job

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