I believe most “PLC faults” are actually panel faults. Not every time—but enough times that I now teach every technician I train to start with the circuit, not the processor.
That belief comes from a pile of my own mistakes, not a sales brochure. I’ve worked in industrial controls since 2014. In 2016, I told a customer their Omron PLC needed to be replaced because a packaging line kept dropping offline. They spent about $1,200 on a new CPU. The fault returned the same week. It turned out to be a worn auxiliary contact in the disconnect feeding the PLC. The original processor was still good. I still keep that service report in my bag as a reminder.
Why does this matter in 2025? Because the industry has changed. PLCs have become more dependable, but a lot of troubleshooting habits have not moved with them. The fundamentals never changed, but the order of operations should have.
Argument one: The processor is rarely the weakest link
I’m not going to tell you Omron PLCs never fail. They can, and I’ve seen a handful of legitimate failures. But in my experience, the CPU is rarely the first thing to break. The same cannot be said for relays, power supplies, terminal blocks, disconnects, and the piles of field wiring attached to them.
The Omron micro PLC line—the CP1L, CP1H, CP1E, and the compact machines that followed—benefits from decades of manufacturing refinement. These controllers run for years in hot panels, vibrating enclosures, and dusty machine cabinets. The components around them face the same abuse without the same engineering margins.
I’ve been called out to maybe 150 control panel faults over the last decade. Maybe 180, I’d have to check the log. But I can count genuine PLC CPU failures on one hand. Loose connections, failed relays, dirty contacts, and miswired switches account for most of the rest.
Argument two: Software diagnostics tell you where to look, not what to fix
Diagnostic tools have gotten better. I am a fan of Omron’s programming software and the built-in diagnostics on the CP1H. But I learned the hard way that a software trace can be misleading if you stop reading at the screen.
In September 2022, I was called to a plant that was convinced their Omron CP1H PLC had a corrupted program. The line dropped out every time they tested the backup generator. I connected to the CPU, opened the program, and found no fatal error. The CPU was running. But the forced trace showed input 0.02 losing its signal for about half a second at the exact moment the generator contactor pulled in.
The trace did not tell me why. It told me where. The difference is important.
I spent the next hour tracing the control power feed. It ran through a 6 circuit manual transfer switch that let maintenance connect the panel to the generator during an outage. Inside that switch, a neutral terminal had worked loose. When the generator alternator kicked in, the vibration opened the connection just long enough for the PLC to see the voltage drop and stop the machine.
The fix took longer to find than to solve. After a proper control panel lockout—full shutdown, lockout, and zero-voltage verification—I pulled the transfer switch cover and retorqued every terminal. There was nothing wrong with the Omron micro PLC. It had responded exactly the way it was supposed to respond: when the input signal disappeared, the machine stopped safely.
Here is the part I teach my team now. If you ask me how to test an alternator with a multimeter, I’ll tell you not to trust a no-load reading. You have to put a load on it and watch for voltage collapse. The same logic applies to a control panel. A PLC can look fine at idle but drop out under real load. You need the physical check, not just the software screen.
Argument three: Old-school electrical skills matter more than ever
There is a common belief that PLC troubleshooting is mostly programming now. I see it in newer technicians: they arrive, open the laptop, and start watching variables. That’s useful. But it does not replace measuring voltage, checking continuity, and understanding what happens when a 6 circuit manual transfer switch has been sitting untouched for two years.
Every time I do a proper control panel lockout and inspect the power side with a multimeter, I feel like I’m doing the same job my mentor did decades ago. The hardware has changed, but the electrical principles have not. That is why I argue that hands-on electrical basics should not be treated as an outdated skill.
What if the PLC really is the problem?
Before you write this off as “he just hates PLCs,” let me address the obvious objection. PLCs do fail. I have replaced a CP1H output, an older CJ power supply, and an input module that had been damaged by an improperly grounded welder. I have also seen rare EEPROM errors that required a reload.
If you spend 20 minutes doing a control panel lockout and measuring the supply voltage, you have not wasted time. You have eliminated the most common causes. The cost of that check is small compared with the cost of replacing a CPU that was never bad.
My updated opinion
I believe a strong troubleshooting process starts with the panel and works its way toward the processor, not the other way around. The industry will keep moving toward more connected, software-heavy automation, and that is a good thing. But PLCs still rely on the physical world to send them clean signals. It is our job to verify that world before blaming the processor.
The Omron PLC was not the problem in most of the calls I have messed up. The problem was usually a bad connection, a bad relay, or a part of the panel nobody wanted to inspect. So check the power first. Lock out the panel, open it, get the multimeter out, and look at the circuit under load. You might save yourself the same embarrassment I gave myself back in 2016.
