Signs of a Failing PLC

Programmable Logic Controllers (PLCs) are designed and manufactured to sustain constant operations in harsh industrial environments, making them rugged computing devices for a whole host of applications. From control systems on the International Space Station to safety devices on naval ships.

PLCs allow us to control and automate low-level operations by sending rudimentary signals as needed to activate sensors, read data, or simply record data in databases. This ensures reliable and consistent control output.

No rugged machine lasts forever, though; machines will tend to fail over time. A clock’s cogs are starting to rust, worn-out gaskets in the air compressor, and electronics are failing due to electromagnetic interference.

Why Should You Care?

PLCs are primarily deployed in mission-critical, predictable control systems.

Factories generate profits from producing goods on a factory line; stopping that line could mean the difference between a loss of thousands to millions of rands.

On mission-critical systems, such as nuclear power plants that were commissioned in the 90’s in the US, PLCs enable predictable control of safety systems. A PLC failure in a nuclear power plant could mean overheating of the cooling system, a lack of pressure regulation in the reactor core, or a failure of an emergency shutdown to trigger. Consequences that could level a city and cause mass fatalities.

In most software disciplines, a slip-up could just mean the user isn’t happy with a graphical glitch. In PLCs, a failure could mean a loss of human life, thousands of rands of profit down the drain.

Signs of Failure

Immediate Signs: LEDs

PLC manufacturers flash their hardware to show red lights on CPUs, communication cards, fuse blocks, and I/O cards. If you see a red light on any of these hardware components, you could be facing general issues with the hardware itself.

It is also advisable to inspect Ethernet ports and communication interfaces for link and activity lights. A lack of communication lights could indicate damaged cabling, failed communication cards, switch failures, or incorrect network configuration.

Some PLCs may also display fault indicators directly on the CPU itself. Intermittent fault lights or faults that temporarily disappear after cycling power can potentially indicate degrading hardware, unstable power supplies, or communication issues.

Burning smells, excessive heat inside the cabinet, or visible discoloration around terminals and connectors can also indicate failing electrical components or damaged hardware.

• Fuse blocks: Check if the fuse was blown and retry the setup. It was just the fuse, and the machine should start up again.
• I/O cards: Inputs changing state without outputs reacting correctly can indicate faults with output cards, field wiring, or PLC logic.
• Analog modules: Incorrect analog readings that do not match field measurements can indicate sensor faults, scaling issues, wiring faults, or analog input card failures.

It is important to remember that the PLC itself is not always the source of the issue. In industrial environments, field devices, sensors, wiring, actuators, and communication equipment are often more exposed to environmental conditions and can fail more frequently than the PLC CPU itself.

Network Issues

PLCs and the devices connected in networks can fail over time, either due to faults, misconfiguration, changes in the network, or failed components.

Often in PLC configurations inside panels, components can fail due to static discharges or electromagnetic interference. You’d need to slowly identify which router, PLC, switch, computer, or cabling is affecting the network.

It can often be traced back to devices not picking up any data, not being able to make any API requests, losing communication with HMIs or remote I/O devices, or being unable to access the network altogether.

Repeated communication dropouts, intermittent Ethernet connectivity, or devices randomly disconnecting from the network can also indicate degrading communication hardware or damaged Ethernet cabling.

In some situations, outputs may remain active when they should not be, or equipment may fail to respond even though sensors appear to be operating correctly. This can indicate communication faults, failed I/O modules, or logic-related issues.

Need assistance troubleshooting Ethernet communication on PLC systems? Check out our troubleshooting Ethernet on PLCs guide.