Top 5 Most Common Causes of PLC Failures and the Warning Signs That Most Teams Miss

There’s a particular kind of dread that settles over a maintenance team when a PLC goes down mid-shift. The line stops. The phone rings. And somewhere in a cabinet full of blinking status lights, something has failed, possibly for reasons that were quietly building for months.

Understanding the real causes of PLC failure matters, not just so you can fix them, but so you can see them coming. The frustrating truth is that most unexpected PLC failures aren’t actually unexpected. They’re the end result of a warning chain that went unnoticed, unread, or unfixed. This post breaks down how each failure actually develops, what it looks like before it becomes a crisis, and what you can realistically do about it.

1. Poor Power Quality: One of the Most Overlooked Causes of PLC Failure

When people talk about power supply failures, they usually mean a PLC that lost power and shut down. That’s the easy case. The harder, more common problem is dirty power which is voltage that’s technically present but unstable, noisy, or laced with transient spikes that most standard meters won’t even catch.

Here’s what actually happens. A variable speed drive on the same panel powers up and injects a voltage spike onto the supply rail. The PLC processor sees a momentary dip or spike, and depending on the unit’s tolerance, it either absorbs it or glitches, triggering a spurious output, dropping a communication link, or logging an unexplained fault. The operator resets it and it runs fine for another week. Then it happens again.

Over time, repeated transient events degrade internal capacitors in the PLC’s power supply module. The failure, when it finally arrives, looks sudden. But the damage was cumulative.

The warning signs most teams miss:

• Intermittent faults that clear on reset with no obvious cause
• Fault logs that show brief CPU errors at consistent times of day (often when a large motor starts)
• I/O modules browning out while the processor stays online

2. Overheating: the Maintenance Assumption That Kills PLC Systems

Heat is one of the most well documented causes of PLC failure, yet facilities still lose controllers to it every year. The reason why is because heat management doesn’t get treated as a regular maintenance job.

A cabinet is designed at installation with a thermal calculation based on the equipment present at that time. Then, over the following years, a new drive may get added, an old relay gets replaced with a larger one, or the factory floor gets hotter in summer and the cabinet filter then gets clogged with dust without anyone noticing. None of these changes individually crosses a threshold. Together, they push the internal cabinet temperature from acceptable to damaging, slowly and without alarm.

Electronic components don’t fail the moment they overheat, they slowly degrade. Electrolytic capacitors lose capacitance. Solder joints develop microcracks. Processor performance becomes erratic at elevated temperatures. When the component finally fails, it can look like a random fault rather than the thermal problem it actually is.

The warning signs most teams miss:

• Cabinet fans that are running continuously when they didn’t used to
• A PLC that faults more often in summer or in the afternoon
• Discolouration or burn marks inside the enclosure (particularly on I/O modules or terminal blocks)
• A cabinet door that’s noticeably warm to the touch

What actually helps: Set a reminder, quarterly at minimum, to physically inspect cabinets for dust on filters, fan operation, and any signs of heat stress. A cheap infrared thermometer can tell you immediately whether the internal temperature is within spec. If you’ve added equipment to a panel since it was originally designed, recalculate the thermal load and add cooling if needed. It’s a small job compared to an emergency replacement.

3. Electrical Noise: the Fault That Hides Behind Everything Else

Electromagnetic interference is one of the most misdiagnosed problems in industrial automation, because its symptoms mimic almost everything else: unexplained faults, dropped network connections, rogue outputs, and corrupted data. If you’ve ever replaced a module because it “seemed faulty” only for the same problem to return, electrical noise may have been the actual culprit.

Here’s the mechanism. EMI couples onto signal or power wiring and induces small voltages that the PLC misreads as real signals. Depending on which wire picks up the interference, you might see false input triggering, garbled serial communications, or erratic analogue readings. The interference source is often something that changed recently: a new drive installed, a welding station moved closer, or a cable run that now shares a trunking with a high current feed.

Grounding is the root issue in most cases. A PLC system with clean, solid grounding at every point has a low-impedance path to dissipate interference. One with poor or inconsistent grounding traps that energy and lets it propagate.

The warning signs most teams miss:

• Faults that correlate with a specific machine starting or stopping elsewhere on the floor
• Analogue sensors that read fine on the bench but fluctuate on-site
• Communication errors that appear sporadically rather than permanently
• Problems that started after a change to the electrical installation, even a seemingly unrelated one

Effective prevention of signal interference involves disciplined cable routing, ensuring signal and power cables do not share trunking or run parallel for long distances. Use shielded cables for communication and analog signals, grounding them at one end, and ensure all earth connections are clean, tight, and properly lugged, as corroded connections may provide a false sense of security. When diagnosing intermittent issues, correlate fault timestamps with other machine activities before replacing components. For grounding references, consult Siemens’ EMC installation guidelines, which are comprehensive regardless of the PLC brand used.

4. I/O Failures: Hardware Gets the Blame, Wiring Is Often the Cause

I/O module failures are the single most common category of PLC fault, but “I/O failure” is really a family of problems, not a single thing. Treating them all as hardware failure means missing the real cause.

Here’s the distinction that matters. When an input or output point stops working, the module itself may be perfectly fine. The fault could be a loose terminal connection, a wire that’s been nicked by cable ties and is shorting intermittently, a field device that’s developed a fault and is back-feeding the module with an incorrect voltage, or a configuration error where the wrong module type was selected after a replacement. True module hardware failure does happen, particularly on older systems where transient overvoltage events have degraded output transistors over time, but it’s less common than the wiring and device issues that surround it.

The other nuance worth understanding: I/O modules on systems that are ten or more years old may simply have reached the end of their service life. Electrolytic capacitors in the module’s power circuitry dry out and lose capacitance. Output drivers degrade. The module starts behaving erratically before it fails completely. If you’re experiencing recurring faults on the same module that clear with a hard reset, ageing hardware is worth considering.

The warning signs most teams miss:

• Faults that only occur after the machine has been running for a while (temperature-sensitive failing components)
• An input or output point that “comes back” when you wiggle the wiring
• A module that works fine when swapped to a test rig but keeps faulting in the field (points to the field side: device, cable, or environment)
• Increased fault frequency on a specific I/O point over the past six months

What actually helps: When diagnosing an I/O fault, work from the outside in. Check the field device first, then the wiring, then the terminal connections, and only then the module itself. Keep a fault log per I/O point if your SCADA or PLC logging supports it. A point that faults 12 times in a year tells a different story than one that’s just developed its first fault. For ageing systems, consider building a list of modules that have exceeded their expected service life and scheduling proactive replacements during planned shutdowns rather than waiting for field failures.

5. Software and Configuration Integrity: the Failure Mode No One Talks About

Hardware failures get most of the attention in discussions about PLC reliability, but software and configuration problems cause their own category of unexpected downtime, and they’re often completely avoidable.

The two most common scenarios. First, a program or configuration backup that doesn’t exist, is out of date, or has never been verified. When a CPU or memory module fails and needs replacement, the team discovers that the last backup was taken before a modification three years ago, or that the backup file is corrupted, or that no one has the software licence to open it. What should be a two-hour hardware swap becomes a multi day recovery exercise.

Second, undocumented changes. A technician makes a temporary modification to the ladder logic to get production running after a sensor fault. The sensor gets replaced, but the logic modification never gets reverted, and nobody documents it. Six months later, a different technician is troubleshooting an unrelated problem and the undocumented change either confuses the diagnosis or actively interferes with the fix.

Firmware is a related risk. Running outdated firmware isn’t just a security issue. It can mean the PLC is running with known communication bugs or memory handling problems that were fixed in later versions. This is particularly relevant when other parts of the system (HMIs, drives, communication modules) are updated while the PLC firmware is left behind, creating version mismatches that show up as intermittent communication drops.

The warning signs most teams miss:

• No one on the current team knows for certain when the PLC program was last backed up
• Backup files exist, but have never been restored and verified
• The system has been modified over the years, but the as-built documentation hasn’t been updated
• The PLC firmware version differs significantly from what the manufacturer currently recommends

What actually helps: Treat your PLC program backup like financial data: version-controlled, tested, and stored in at least two places. After any programme change, the backup should be updated before the maintenance crew leaves the site. Schedule a firmware review annually as part of your maintenance plan. You don’t have to update every time, but you should make a conscious decision rather than ignoring it. If you’ve inherited a system with an unknown modification history, a proper audit before something goes wrong is far less painful than a recovery after.

The Pattern Behind All Five Causes of PLC Failure

Looking across these failure modes, one thing stands out is the actual failure event is almost never where the problem started. A PLC that goes down due to heat usually has a clogged filter and a rising fault frequency for months beforehand. An I/O module that fails was often preceded by intermittent faults on that same point. A software recovery crisis is the downstream result of a backup process that was neglected years earlier.

The systems that avoid unexpected downtime aren’t necessarily the ones with the newest hardware. They’re the ones where someone is paying attention, running structured maintenance routines, keeping records, and treating small warning signs as real information rather than inconveniences to be reset and forgotten.

That’s the work. And it’s more achievable than most teams realise, once there’s a process behind it.

At Mizen Automation, we specialise in repairing PLC equipment and sourcing obsolete and hard-to-find parts. So whether your system has already gone down and you need a fast turnaround repair, or you’re keeping an ageing installation running and struggling to find components that are no longer in production, we can help. Get in touch with the team and let us know what you’re dealing with.