5 Signs Your PLC System Needs an Urgent Upgrade

Your PLC has been running reliably for 15, 20, even 30 years. It still works. Why spend money replacing something that works? Because "still working" is not the same as "safe, maintainable, and future-proof." Here are five signs that your PLC system has crossed the line from "aging but reliable" to "a liability waiting to happen."

Sign 1: You Are Buying Spare Parts on eBay#

When the manufacturer stops producing spare parts, you enter the secondary market — refurbished parts suppliers, eBay, surplus dealers. The prices tell the story:

• A new S7-300 CPU from Siemens (when available): ~€1,000
• The same CPU from a surplus dealer today: €2,000–5,000
• An S5 CPU 944 (no longer manufactured since 2003–2006): €3,000–8,000 if you can find one at all

The real danger is not the price — it is the lead time. When your CPU fails at 2 AM on a Friday and the surplus dealer needs 2–3 weeks to ship a refurbished unit, your production line stands still. At €5,000–50,000 per hour of downtime (depending on your industry), even one failure event can cost more than the entire migration.

What this looks like in practice: Your maintenance team has a shelf with "emergency spares" for the PLC — parts hoarded over years, bought whenever they appeared on the market. If that shelf exists, your PLC system is overdue for an upgrade.

What to do: Inventory your spare parts. Calculate the cost of a 48-hour unplanned downtime. Compare it to the migration cost. The math almost always favors migration.

Sign 2: Your Programming Software Runs on Windows XP#

STEP 5 runs on Windows XP. STEP 7 V5.4 runs on Windows 7. Both operating systems have reached end-of-life — Windows XP since April 2014, Windows 7 since January 2020. Running these on a network-connected PC is a cybersecurity risk that no IT department should accept.

The practical problem is deeper than security. The laptop that runs STEP 5 is itself aging. The hard drive is 15 years old. The serial port adapter uses drivers that barely work. If that laptop dies, you lose access to your PLC program — not the program itself (it is in the PLC), but the ability to read, modify, and troubleshoot it.

What this looks like in practice: There is one specific laptop in the maintenance office that "must not be touched." It runs Windows XP, has a STEP 5 sticker on it, and someone has written "DO NOT UPDATE" on a piece of tape on the lid. Everyone knows about it. Nobody talks about the risk.

What to do: If your programming environment depends on an unsupported OS, start the migration conversation now. TIA Portal runs on current Windows versions, is actively maintained, and will be supported for decades.

Sign 3: Only One Person Understands the Program#

Every plant has them: the engineer who wrote the original PLC program 20 years ago, or the one maintenance technician who has kept it running through sheer experience and institutional knowledge. When that person retires, changes jobs, or is simply on vacation during a crisis — the plant is exposed.

What this looks like in practice: When a PLC problem occurs, everyone's first question is "Is [name] available?" If the answer is no, troubleshooting takes 5–10× longer because nobody else can read the undocumented AWL code with cryptic addresses.

The demographic reality: Engineers who learned S5 programming in the 1980s are reaching retirement age now. This is not a future problem — it is happening right now. Every month, the available pool of S5 expertise shrinks permanently.

What to do: Start a knowledge capture process immediately (see our knowledge loss guide). Use PLCcheck Pro to generate documentation from the existing code — before the expert leaves.

Sign 4: Maintenance Costs Are Rising While Nothing Is "Broken"#

The insidious nature of aging PLC systems is that costs rise gradually. No single invoice triggers alarm. But when you add up the numbers:

• Troubleshooting takes longer: Diagnosing problems on a 20-year-old system without modern diagnostics takes 2–5× longer than on a current system. S7-1500 provides plain-text error messages, web-based diagnostics, and trace recording. S5 provides a blinking LED and a hex code.
• Every modification is expensive: Changing a recipe, adding a sensor, or adjusting a sequence requires an expert who knows STEP 5 and has access to a working programming device. Hourly rates for S5 expertise are €120–180/h because so few people still have the skill.
• Workarounds accumulate: Instead of proper program modifications, the plant invents workarounds — additional relays, manual switches, Excel spreadsheets replacing PLC-based counting. Each workaround adds complexity and reduces reliability.

What this looks like in practice: You notice that maintenance spending has increased by 15–30% over the past 5 years, but no one can point to a single cause. The cause is system age — distributed across dozens of small inefficiencies.

What to do: Track maintenance costs per machine. Compare PLC-related costs year-over-year. Present the trend to management — a 20% annual increase compounds to a doubling in 4 years.

Sign 5: Your Machine Cannot Talk to Anything Modern#

Legacy PLCs were designed for standalone operation. They control a machine — period. But modern manufacturing requires data:

• MES integration: Production orders, batch tracking, quality data
• Energy monitoring: Knowing which machine consumes how much power
• Predictive maintenance: Vibration data, temperature trends, cycle time analysis
• Remote access: Diagnosing problems without driving to the plant at 3 AM

S5 PLCs have no Ethernet, no OPC UA, no web server. S7-300 has PROFINET but limited data connectivity. S7-1500 provides all of the above out of the box — OPC UA, web server, integrated diagnostics, cloud connectivity.

What this looks like in practice: You have invested in a new MES system, but your oldest machines are "dark" — they produce parts but provide no data. The plant manager gets dashboards for 60% of the production lines and blank screens for the rest.

What to do: If your Industry 4.0 / smart factory initiative is being held back by old PLCs, migration is not just a maintenance decision — it is a strategic investment in data-driven manufacturing.

The Question Is Not "If" But "When"#

All five signs point in the same direction: the cost and risk of NOT migrating increases every year, while the cost of migration stays roughly constant (adjusted for inflation). The optimal time to migrate was 3 years ago. The second-best time is now.

Start With Understanding What You Have#

Before committing to a migration budget, understand the scope. PLCcheck Pro analyzes your existing PLC programs and delivers:

• Complexity assessment: How much engineering effort does migration require?
• Risk inventory: Which systems are most critical and most at risk?
• Code documentation: What does the program actually do — in plain language?

This data is the foundation for every business case, vendor quote, and project plan.

Get your free code analysis →

Frequently Asked Questions#

My PLC still works fine. Why should I upgrade?

"Works fine" today does not mean "works fine" tomorrow. The five signs above are leading indicators — they predict future failures, not current ones. The goal is to migrate proactively (planned, budgeted, minimal disruption) rather than reactively (emergency, expensive, maximum disruption).

Which sign is the most urgent?

Sign 1 (spare parts) and Sign 3 (single expert) are the most time-critical because they involve resources that are depleting irreversibly. You cannot un-retire an expert or un-discontinue a spare part.

How do I convince management?

Use Sign 4 (rising maintenance costs) and the ROI calculation from our business case guide. Executives respond to numbers, not to technical arguments about PLC generations.

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Maintained by PLCcheck.ai. Last update: March 2026. Not affiliated with Siemens AG.