The top reasons why PLCs fail and how you can prevent it


Just like all machinery and electronic equipment, PLCs (programmable logic controllers) can go wrong. It can be very difficult to identify why a PLC might fail but not only that, downtime can be very costly. So let’s look at why a PLC might breakdown and how we can avoid this.

An easy way of diagnosing faults in a PLC is to use a copy of the PLC software, a laptop, a multimeter and a programming lead.

The most common reasons PLCs fail are:

  • I/O modules and field devices
  • Power supply issues
  • Interference
  • Networks and communications
  • Heat

Let’s see why these elements might cause problems.

I/O Modules and Field Devices

The main causes of PLC failure are down to I/O modules, field devices or power supply issues. 80% of PLC failure is caused by one of these faults. These faults show obviously by either stopping suddenly or being irregular in its performance. When this happens, the engineer diagnoses where the sequence has stopped by interrogating the software ‘On-line’, this is so that they can trace the problem to a specific I/O module and input or output point.

Tracing the fault to a specific point helps the engineer find the root cause. This could be down to anything, such as, PLC configuration failure, loose terminal block, tripped circuit breaker, issues with wiring or even a failure of a 24VDC supply.

Power Supply Issues

The main reason PLCs can fail can be because of loose or corroded connections. A lot of companies usually have redundant power systems or they will install uninterruptible power supplies (UPS) in order to keep things running in case of a mains power failure. This provides control of items to maintain its safe operation.

When power is lost, it is possible that the PLC could lose its memory. This will affect all operational programs, to avoid this a PLC sometimes has a backup battery. A PLCs backup battery ensures the device restarts correctly.

It is important that you backup all PLC Software frequently and store it safely and securely. If software is not backed up on a regular basis and something does go wrong it will make it very difficult to go on as normal, making a small issue much greater.


Industrial environments that contain a variety of electrical equipment commonly experience electromagnetic interference (EMI) and radio frequency interference (RFI). Anything from small radio transmitters to huge motors can cause interference. It is very important for industrial environments to control electrical noise as it can cause unusual behaviour, intermittent faults and PLC failure.

There are lots of different ways to reduce downtime caused by electrical noise, such as:

  • Relocate sensitive equipment
  • Grounding
  • Shielding cables between sensitive equipment
  • Segregate systems with high power components and add barriers

Engineers need to make sure the physical network infrastructure is correctly installed and terminated correctly. They regularly install firmware patches in order to maintain the security and reliability of the operation.


It is vital that the environment is suitable for the equipment and control systems. Without air filtration components in the cabinets, it can cause insufficient airflow. This can cause components to overheat, causing downtime.

If equipment goes over the manufacturers recommended temperature it is likely that it will fail. High humidity also causes condensation on electrical products, this can lead to PLC failure. To prevent this, make sure you think carefully about where a control panel is located when installing it and/or use panel-cooling systems like many other industrial plants.

Network and Communications


It is vital that most PLCs communicate with devices such as HMIs and other equipment. Sometimes communication can be lost between devices, this results in immediate downtime. Engineers can reduce communication failures by making sure that the physical network infrastructure is installed and terminated correctly. It is also important that when adding more devices they are suitable for the purpose and that firmware patches are installed frequently to maintain reliable and secure operation.

Managing the Risks of PLC Failure

It can be very easy for companies to manage the chance of PLC control system failure. By following simple steps, engineers can prevent risks. It is important that regular visual inspections take place in order to identify overheating or electrical noise problems in its early stages.

Batteries and UPS systems should be tested frequently. This will ensure that systems are reliable and operate continuously even if a power fault did occur. Other important things to check are:

  • Wiring integrity
  • Grounding
  • Terminals
  • Field devices
  • Ethernet and other industrial networks.

Plant managers should backup software and install firmware patches regularly.

Obsolescence Management

Finally, a major factor is obsolescence management. PLC manufacturers often update and change their range. It is important that you have replacements for devices and equipment that are several years old. Companies can manage this by themselves or by hiring someone to identify which areas are most likely to fail and put a plan together to manage the risks.

Although all these factors do help reduce the risk of downtime, it can still occur. This is why it is vital that PLC software is regularly backed up, this is so that normal functions can resume quickly if downtime does occur. Another way of making the system more secure is upgrading firmware.

Here at Northern Industrial, we have a wide range of PLCs available from manufacturers including, Siemens, Beckhoff and Allen Bradley. For more information please contact our friendly sales team [email protected]. We can also help you with obsolescence management with our free report service, find out more at


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