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Common PLC CPU Failures

PLC’s CPU Functions

Programmable Logic Controllers are digital electronic devices that are mainly used as industrial control systems. PLCs contain a programmable memory that stores a series of user-defined instructions. They function by continuously monitoring the state of input devices and executing the stored program logic so as control the state of connected output devices. Since modern PLCs have increased in functionality and are very cost-effective, PLCs are now becoming mainstream in the sector of industrial automation; as fundamental systems that can monitor and control both complex processes and individual equipment.

The main component of the PLC system is the Central Processing Unit (CPU) which acts as the brain of the PLC. It hosts the PLC logic, communications, and memory. Similar to the human brain, the CPU controls every function and circuitry of the PLC; from receiving input signals to executing logic functions, and sending control commands to the outputs. It performs the following operations: (i) Updating inputs and outputs. (ii) Performing simple and complex arithmetic and logic operations involved in a PLC control system. (iii) Communicating with memory- since the PLC data and programs are stored in the memory; so, whenever the PLC is operating, its CPU reads the contents of the memory locations while making necessary changes. (iv) Scanning application programs; this function allows the PLC to execute the application program as specified by the user/programmer. (v) Communicating with a programming device; the CPU transmits data and control information between itself and the programming device (programming terminal).

Note, the CPU of any PLC is controlled by an Operating System(O/S) software. The O/S entails a group of supervisory programs loaded and stored permanently in the PLC memory by the PLC manufacturer. When your PLC’s CPU overheats due to poor current flow, or when you operate the PLC system within conditions that are not as recommended by its manufacturer or if it’s used to carry out functions for which it wasn’t designed, then the CPU is likely to fail completely. This article outlines some of the most common PLC CPU failures. It also provides a basic approach to determining the root cause for the outlined problems and ways in which each failure mode can be mitigated before the CPU fails completely.

1. Data/Program Loss

Power outages in themselves do not usually damage your PLC hardware, but they can lead to loss of your programmed logic and PLC configurations. The PLC memory (ROM and RAM) is located within the CPU. In that regard, the ERPROM (Erasable Programmable Read-Only Memory) within the CPU stores the PLC logic, configuration settings, process setpoints, and real-time clock.

In the event of power failure, the contents of the CPU’s non-volatile memory will be lost. Hence, your PLC will not function as intended as the CPU will have no programmed logic to execute unless you write the program again and load it into the PLC. To prevent this, a PLC battery is used to provide backup power to retain the CPU’s memory contents in the occurrence of power outages. The PLC battery should be serviced regularly and replaced accordingly, to ensure that it has adequate voltage to back up the processor’s non-volatile memory.

2. Excessive Heat Failure

Overheating is the root cause of more than 95% of all CPU failures. Whenever it gets too hot, you find that your body becomes sluggish- even the slightest movements may be exhausting. In the same way, the functionality of PLC CPUs is also affected by high temperatures. Generally, a working CPU generates a lot of thermal energy(heat) as it processes huge amounts of data at high speeds. Most PLC CPUs are equipped with inbuilt cooling systems (cooling fans) which regulate the CPU’s internal temperatures.

But sometimes these cooling fans may become inefficient with age or they may get clogged with dust. If the cooling system is not functioning properly, the heat levels within the CPU module will definitely go beyond the normal threshold; ideally, the operating temperature of a PLC CPU shouldn’t exceed 50 oC. If the CPU gets exposed to too much thermal energy, permanent damage or failure will result.

The best way to prevent CPU failures due to excessive heat is through proper maintenance of the cooling systems and close monitoring to note when the CPU is starting to heat up. This will ensure that the PLC CPU is not being pushed to its limits, causing it to overheat. Also, you should ensure that your PLC system is operating within the temperature levels recommended by its manufacturer, that way its CPU won’t be at risk of heat-related problems.

In addition, if the PLC system is located close to heat-emitting equipment, its CPU will be at risk of heat-related failure if it isn’t properly safeguarded. As a general rule, in facilities where HVAC (Heating, Ventilation, and Air Conditioning) systems are used, the cooling system should be set to a level that will keep the area around the PLC system at relatively low temperatures. Moreover, any equipment or machinery that generates heat should be kept at a considerable distance from the PLC system.

3. Corrupted Memory

The PLC memory is located within the CPU, and it stores the logic involved in program execution. PLC CPUs use two types of memory namely Read Only Memory (ROM) and Random-Access Memory (RAM). The PLC memory in the CPU can get corrupted by external factors such as Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).

EMI interferences are a form of electrical noise which results from large motors or other inductive machines placed close to the PLC system, EMI can also be induced by lightning strikes. RFI results from antennas and handled radio transmitters used by maintenance personnel, in close proximity to the PLC system. The handheld radios emit strong radiofrequency radiations which disrupt and interfere with any unprotected electronic equipment including the PLC system. Generally, EMI and RFI are known to introduce operational faults or errors that can degrade the performance of the equipment, and in the worst-case scenario, they may cause complete failure of the equipment.

When we say that EMI or RFI frequency interferences corrupt the PLC memory, it means they render the code unreadable by the CPU. This results in errors when the CPU is reading or writing the memory. For example, the RAM memory enables the PLC CPU to keep things functioning smoothly and quickly. So, if the CPU is not able to access the RAM adequately then your PLC will slow down, or it may freeze during complex operations, or stop working completely. Generally, the faster the RAM accessibility, the faster the processing speed of the PLC CPU.

In addition to introducing errors in reading or writing data, EMI and RFI may interfere with data or information exchange between the CPU and other components of the PLC system, leading to loss of communications, analog inputs, or outputs reporting incorrect values, and discrete inputs or outputs triggering unexpectedly. The PLC CPU will thus become efficient, and continued exposure to electromagnetic and radio frequency radiations may cause the CPU to fail completely resulting in significant downtime and costly damages.

Hence, any potential source of EMI or RFI should be contained or prohibited from the vicinity of the PLC system. The PLC system should also be placed in a properly grounded and shielded metal enclosure designed specifically to reduce EMI. Moreover, zinc-plated backplanes should be used with PLCs to further reduce their susceptibility to electrical noise. Alternatively, it is important to keep all PLC data and program backups away from EMI and RFI sources, and occasionally verify if the program in use is still correct and comparable with the backup copy.

4. Failures related to Power Surges and Voltage Spikes 

The main cause of PLC logic board failure is power surges and voltage spikes. As previously stated, power outages usually don’t damage the PLC CPU themselves but they are often accompanied by harmful power surges and voltage spikes. A power surge is a sudden increase in voltage mostly caused by power interruptions and changes in the electrical current draw for equipment using the same power supply. It’s the severity and timing of the power surge that can result in CPU problems.

On the other hand, voltage spikes are similar to power surges but they are more severe and pose the largest potential problems to PLC CPUs. They are typically the byproduct of more drastic events such as electromagnetic pulses (EMPs), tripped breakers, short circuits, and other power supply failures.

PLC CPUs are highly susceptible to voltage spikes, transient voltages, and power surges as PLC systems are mainly used in factory floors with large motors and other types of inductive loads. For this reason, a separate PLC power supply along with additional surge protectors (filter protection schemes) and surge suppressors are essential components of PLC systems, to enhance the survivability of the CPU in the occurrence of voltage spikes, or power surges, and transient voltages.

Hence, the voltage spikes and power surges that destroy a PLC’s CPU are comparatively rare, but they do sometimes happen if the power supply unit gets old enough to transmit the surges to the PLC components. If this happens, the current flowing through the PLC’s motherboard will definitely exceed the set breakdown voltage, destroying the available electrical insulators and other sensitive circuitry like CPU circuitry. Once the PLC circuit board is damaged, the CPU will fail completely as it sits on the motherboard (logic board).

5. Transistor Failure

PLC CPUs being digital logic processors they are made of transistors containing Silicon elements. There are two common types of transistors that you will find in most modern PLC CPUs, namely n-MOSFET’s and p-MOSFET’s (n-type and p-type Metal Oxide Semiconductor Field-effect Transistors). Transistor failure occurs when excessive heat causes the microscopic impurities within the silicon element of the transistors to diffuse and change the operating parameters of the CPU transistors. This may eventually damage the CPU transistors, and whenever a critical transistor fails the CPU may die suddenly.

It only needs one transistor to fail before a PLC CPU stops functioning, but you may ask why this doesn’t happen more often. Well, it’s because a modern PLC CPU contains millions of transistors and the effect can be different depending on the location of the failed transistor in the CPU. For example, if a transistor fails in the Arithmetic and Logic Unit (ALU) the performance of the PLC CPU may not be affected, and the failure may go unnoticed until a particular instruction is executed; given that some instructions in a PLC program are executed less frequently.

Note, sufficient cooling of the PLC system is very necessary as heat generation is an unavoidable consequence of simply operating the CPU transistors, otherwise failures will be more often.

6. Compatibility Failure

A PLC CPU may malfunction or fail whenever the internal status of the application code conflicts with the outer environment. For instance, when an incompatible I/O module is connected to the PLC system. Or maybe an upgrade has taken place, and the application code being executed by the CPU fails to comprehend these changes.

In any case, the CPU can only control what it is programmed to understand. Hence, if a PLC CPU can’t comprehend the command signals it receives from an incompatible input device, it will not be able to actuate those commands. This will result in its failure to function. Therefore, before any changes or updates are made to a PLC system, they should be fully tested for compatibility with the PLC CPU being used.

Symptoms of PLC CPU Failure

The following are signs through which you can identify a failed PLC CPU.

A) Booting Issues: If the CPU of your PLC system has failed, it won’t go through the normal process of booting. PLC systems have diagnostic indicators which show the status of the CPU. They include Power OK, Memory OK, and Communications OK LED indicators. 

The first thing you should do whenever you’re starting the PLC system is to ensure that its power supply is providing adequate power. But if the PLC doesn’t start up even with appropriate power, the problem could be a case of CPU failure. In such a case, check if the diagnostic LED indicators on the CPU module are showing fault in either communications or memory. If any of the indicators light up, then you may need to replace the CPU of your PLC.

B) Frequent Shutdowns: As previously mentioned, a PLC CPU subjected to excessive heat may possibly undergo failure. CPUs generate heat as they process large amounts of data at high speeds. This heat is normally dissipated by cooling fans, but if the fans get clogged with dust; they are unable to expel the generated heat out. As a result, the CPU gets overheated above the maximum threshold. So, to prevent further damages to the CPU, the motherboard takes necessary precautions by shutting down the PLC system. Hence, frequent PLC shutdowns indicate that the CPU is heating up and is highly likely to fail. 

C) Physical damages: Generally, you can identify a damaged CPU whenever you’re carrying out preventive or corrective maintenance of your PLC system. A CPU that has been exposed to excessive heat for a long time gets permanently damaged. From which you can notice burn marks around the socket of the CPU and charred marks on the CPU. This type of failure is severe and irreversible, all you can do is simply replace the CPU module. 

D) Freezing: Random freezing of the PLC system when in use is another symptom that results from CPU failure. In this case, freezing means that the PLC isn’t responding to any input signal. This can happen once you have started the program scan or running the PLC; causing you to restart the system. 


The CPU is the most important component of a PLC system. It stores the programmed logic, memory, and communications. Simply put, the CPU is the brain of the PLC as it performs all the logic operations and issues all control commands. Hence, any type of CPU failure will cause the entire PLC system to stop functioning. This article has outlined the most common PLC CPU failures that are mainly caused by power outages, excessive heat, power surges, and voltage spikes, electromagnetic and radio frequency interferences, as well as poor maintenance of cooling fans.

As noted, most of these PLC CPU failures are severe and irreversible, and they can only be resolved by replacing the CPU module. This results in significant downtime of the PLC controlled system or equipment, and may result in huge losses in terms of costly repairs and lost production. Hence, it is important you eliminate the potential causes of PLC CPU failures and ensure that the PLC is operating within safe ranges. Also, whenever you notice a symptom of CPU failure is sure to address it as soon as possible, to prevent further damages to the PLC CPU. For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on November 22nd, 2021 and is filed under PLC, Troubleshooting, Uncategorized. Both comments and pings are currently closed.

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