Why is My PLC Throwing Unexpected Errors?

When a PLC fails, what’s really to blame? A bad program? A wiring issue? A dying processor? Some faults point to minor software hiccups. Others signal deep-seated hardware failures that no programming tweak will fix. Whatever the case, unexpected issues don’t just appear out of nowhere. They’re often preceded by errors.

Resolving PLC errors requires an understanding of how they originate and the modes of tracking them back to their source. Here’s how to approach the problem.

1. Look at the error codes first

A PLC won’t just stop working — and even if the failure is sudden, there’s a data trail you can immediately start to follow. System logs, diagnostic indicators, and error messages all provide insight into what went wrong:

• Processor faults such as a timer error or memory checksum failure could mean a bad CPU or firmware issues.
• I/O module errors could be a signal to check for blown fuses, disconnected sensors, or faulty input modules.
• Communication failures often trace back to bad cables, failing network switches, or misconfigured protocols.
• Power supply issues can cause intermittent PLC resets, unexpected shutdowns, or corrupted data.

2. Rule out software and programming mistakes

Sometimes an error isn’t a true failure — it’s just the PLC responding as-programmed. Bad ladder logic such as conflicting rung conditions can cause unexpected results. Likewise, a program that relies on real-time feedback might fail if the scan cycle lags behind physical inputs. Even something as simple as a mistimed delay or counter rollover can throw off an entire process. Testing the logic with live monitoring helps catch these issues in real time. If the program runs correctly but errors persist, the problem likely lies elsewhere.

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3. Inspect hardware for physical damage

When software checks out but errors continue, it’s time to look at the hardware. A physical inspection combined with multimeter testing can reveal problems before they escalate into total system failure. Look for:

• Loose or corroded wiring: Even a slight loss in signal continuity can cause false faults or dropped inputs.
• Failing relays and contactors: Mechanical wear in relay coils or contactors can lead to intermittent failures.
• Overheating: Heat shortens the lifespan of components. Check for blocked ventilation or failing cooling fans.
• Damaged I/O modules: If inputs or outputs randomly stop responding, a failing module may be at fault.

4. Check the power supply and grounding

A PLC needs stable, clean power. Power-related issues are some of the hardest to diagnose, but they account for a significant number of PLC failures. Test and observe the following:

• Use a voltage meter to confirm the PLC is getting a consistent supply. If readings fluctuate, check power sources.
• Look for electrical noise from nearby variable frequency drives, large motors, or improperly shielded cables.
• A floating or poorly grounded PLC may experience erratic faults that seem random but follow no clear pattern.

5. Know when to replace failing components

Some failures are a sign that a component is at the end of its life. A processor that crashes repeatedly could be failing or an I/O module that intermittently drops signals may need replacement. A power supply that resets unexpectedly is probably dying. The good news? Component replacements can be simpler and less costly than many other resolutions — especially when performed by a partner like GES.

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