PLC Obsolescence: The Spare Parts Crisis in Industrial Automation

On October 1, 2025, Siemens stopped manufacturing the SIMATIC S7-300 — after more than 30 years as one of the most widely deployed PLCs in the world. More than 267 module types are now only available as spare parts with diminishing supply. The S5 series lost manufacturer support even earlier, on September 30, 2020. For plants still running these controllers, the spare parts situation is no longer a future concern — it is the current reality.

The Siemens Lifecycle: How PLCs Die Slowly#

Siemens manages product end-of-life through a defined lifecycle with specific milestones. Understanding these milestones is essential for planning:

PM300 — Active Product: Full production, full support, recommended for new projects. All S7-1500 and S7-1200 modules are here today.

PM400 — Phase-Out Announced: Siemens formally announces discontinuation. Marketing shifts from new sales to spare parts. For S7-300, this was announced October 1, 2023.

PM410 — Production Stopped: New production ends. Only existing stock and spare parts remain. For S7-300, this happened on October 1, 2025. From this date, every S7-300 module sold comes from existing inventory — once it is gone, it is gone.

Spare Parts Window: Siemens commits to spare parts availability for 10 years from the PM400 date, not from PM410. For S7-300, this means spares are guaranteed until approximately October 2033. After that, availability depends on remaining stock.

Where We Stand Today (March 2026)#

The Price Curve: What Happens After Production Stops#

Spare parts pricing follows a predictable pattern after production stops — a pattern we have already observed with the S5 series:

Years 0–2 after PM410: Prices rise 20–50% as supply tightens and distributors adjust pricing. Delivery times extend from days to weeks.

Years 3–5 after PM410: Prices rise 100–300%. Specialty brokers and surplus dealers become the primary source. Counterfeit and refurbished parts enter the market, introducing quality risks.

Years 5–10 after PM410: Some modules become effectively unobtainable at any price. Lead times of months are common. Emergency sourcing during a production-critical failure becomes a matter of luck, not planning.

S5 example (6 years after EOL): An S5 CPU 944B that cost approximately €500 new is now listed at €3,000–8,000 on the secondary market — when available at all. S5 communication processors (CP 143) are essentially extinct.

The Three Risk Scenarios#

Scenario 1: The Predictable Failure#

An electrolytic capacitor on a 25-year-old S5 power supply dries out. The power supply fails. No spare on the shelf. The surplus dealer has one — in 3 weeks. Production stands still for the duration.

Cost: Spare part €2,000 + downtime (3 weeks × 40 hours × €10,000/h) = €1,202,000 total impact.

This scenario is not hypothetical. Every S5 power supply has electrolytic capacitors. Every electrolytic capacitor has a finite lifespan. The question is not "if" but "when."

Scenario 2: The Cascade Failure#

A lightning strike damages the S7-300 CPU and two I/O modules simultaneously. Siemens can deliver one replacement CPU from spare stock — in 6 weeks. The specific I/O module variant is no longer available at all because it was a low-volume type.

Consequence: The machine cannot be restored to its original configuration. Rewiring to a different I/O module type requires program changes. No one in the plant can modify the S7-300 program because the STEP 7 Classic programmer left 2 years ago.

Scenario 3: The Expansion Impossibility#

The plant wins a new contract that requires doubling production capacity. The additional production line needs 8 more S7-300 I/O modules. Siemens cannot supply them — production has stopped. Surplus dealers have 3 of the 8 needed. The expansion cannot proceed without migrating to S7-1500.

Consequence: The migration that was "planned for next year" becomes an emergency project — at premium cost and compressed timeline.

Mitigation Strategies (Ranked by Effectiveness)#

Strategy 1: Migrate to S7-1500 (Best Long-Term Solution)#

Replace the aging PLC with a currently manufactured system. Full Siemens support, modern features, spare parts guaranteed for decades.

Investment: €10,000–50,000 per machine (hardware + engineering). Result: Problem eliminated permanently.

Strategy 2: Strategic Spare Parts Stocking#

Buy critical spare parts now, while they are available and affordable. Focus on: CPUs, power supplies, communication processors — the components most likely to fail and hardest to replace.

Investment: €5,000–20,000 per system for a 5-year buffer stock. Limitation: Buys time but does not solve the problem. Components degrade in storage (electrolytic capacitors, battery-backed memory). Stocking is a bridge, not a destination.

Strategy 3: Third-Party Repair Services#

Companies like Eichler GmbH, Foxon, and others offer repair services for failed S5 and S7-300 modules. They replace defective components on the board level and return the module to service.

Investment: €200–1,500 per repair depending on module type. Limitation: Repair is only possible if the module is physically repairable (no burned traces, no cracked ICs). Turnaround time: 1–4 weeks. Not suitable for emergency situations.

Strategy 4: Emulation / Adapter Hardware#

Replace the CPU with an emulator that runs the original program on modern hardware. See our detailed comparison of emulators vs. full migration.

Investment: €2,000–5,000 per CPU. Limitation: The S5/S7-300 program remains unchanged — no modern features, no documentation improvement, new vendor dependency.

The Math That Changes the Conversation#

For every plant running S5 or S7-300 systems, this calculation applies:

Annual expected downtime cost = P(failure) × hours_down × cost_per_hour

Where:
  P(failure) for 20-year-old system ≈ 15–25% per year (increasing)
  hours_down with spare parts available ≈ 8–24 hours
  hours_down without spare parts ≈ 120–500+ hours
  cost_per_hour = your production value (typically €5,000–50,000)

With spare parts on shelf: 20% × 16h × €10,000 = €32,000/year expected cost. Without spare parts: 20% × 240h × €10,000 = €480,000/year expected cost.

The difference — €448,000/year — is the annual value of having a functioning PLC system. It vastly exceeds any migration budget.

How PLCcheck Pro Fits Into Your Obsolescence Strategy#

Whether you choose migration, stocking, or a combination, you need to understand what you have:

• Code analysis: PLCcheck Pro documents every block, every address, every timer — creating the knowledge base you need regardless of strategy
• Complexity assessment: Accurate engineering-hour estimates for migration budgeting
• Risk ranking: Identify which machines are most exposed to spare parts risk
• Migration readiness: When you decide to migrate, the documentation is already done

The worst position is having no plan and no documentation when a critical failure occurs. PLCcheck Pro eliminates the second problem immediately and helps solve the first.

Analyze your PLC programs now →

Frequently Asked Questions#

How long will S7-300 spare parts really be available?

Siemens guarantees spare parts until approximately October 2033 (10 years from PM400 announcement). In practice, popular modules may remain available longer through third-party dealers. Rare or low-volume modules will disappear much earlier. The safest assumption: plan as if your specific module will be unavailable by 2030.

Should I stockpile spare parts or migrate?

If migration is planned within 3 years, stocking critical spares as a bridge makes sense. If migration has no concrete timeline, stocking is just delaying the inevitable while the stocked parts age in storage. The best approach: stock the most critical spares AND set a migration date.

My S5 system has been running for 30 years. Why would it fail now?

Electrolytic capacitors, backup batteries, relay contacts, and connector pins all have finite lifespans. A 30-year uptime record means you have been fortunate, not that the system is immortal. The failure probability increases exponentially with age — each additional year without replacement is a gamble with higher stakes.

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