The S5 Restart Running Problem: Two Ways to Approach It
If you've ever watched a Siemens S5 PLC restart itself in the middle of a production run — no warning, no visible fault code, just a silent reset — you know how frustrating this issue is. It's the kind of fault people call "intermittent," which is engineer-speak for "we have no idea what's causing it."
Over the past decade, I've reviewed hundreds of fault reports as a quality compliance manager in the industrial automation space. The S5 restart running problem shows up more often than you'd expect for a platform that's been out of production for years. And when it does, I see engineers split into two camps:
Approach A: Systematic diagnosis. Read the PLC logic, check the interrupt stack, verify power and grounding, form a hypothesis, and test it.
Approach B: Parts swapping. Replace the power supply, memory card, battery, or CPU until the restarts stop. Or don't.
This article compares both approaches across three dimensions that matter to any plant engineer: diagnostic accuracy, total cost, and long-term reliability. If you're dealing with an S5 that keeps restarting, this framework should help you decide where to start.
Diagnosis: Let the PLC Logic Speak vs. Swapping Parts Blind
Here's the thing I've learned from reviewing thousands of fault tickets: intermittent restarts are rarely caused by the component people suspect first. I once audited a plant where the maintenance team had replaced the power supply, two I/O cards, and the entire CPU on an S5-115U over three weeks. The restarts kept happening. They'd spent around $4,800 on parts and maybe 60 man-hours. The actual cause was a corroded backplane connector — something no amount of parts-swapping would ever catch.
I assumed they'd checked the basics before diving into hardware replacement. Didn't verify. Turned out they'd never once read the diagnostic buffer — because nobody on that shift knew the S5 had one.
The systematic approach uses the PLC's own tools. According to Siemens Industry Online Support, the S5 diagnostic buffer is the first place to look when troubleshooting CPU restart issues. It records restart events with timestamps and fault codes. The interrupt stack, accessible through STEP 5, goes deeper: it shows you whether the restart was triggered by a cycle time violation, a programming error, a hardware fault, or a watchdog reset.
Understanding Siemens PLC logic — especially how the S5 scans its program, handles interrupts, and manages the watchdog timer — is what lets you interpret these clues correctly. A few diagnostic questions that have served me well:
- Does the restart correlate with specific machine movements? If yes, think EMI or grounding.
- Does it happen at power-on only? Look at the power supply.
- What does the interrupt stack say? Addressing errors point to memory issues; cycle time violations point to program or data corruption.
In my experience, systematic diagnosis identifies the root cause in one or two site visits. The parts-swap approach averages three to five visits before someone eventually runs the diagnostics. That's not a small difference — it's the difference between a day of downtime and a week.
Cost: The $115 Fix That Turned Into a $9,000 Problem
Let me address the "$115 PLC restart running problem in Siemens S5" search directly, because I know someone out there is looking for a cheap part to solve this.
In Q1 2024, a packaging plant reached out about an S5-95U with intermittent restarts. A forum post had suggested replacing the 24V DC power supply — around $115 for a compatible third-party unit (based on online prices at the time; verify current rates). They ordered it, installed it, and the restarts continued. Then they replaced the memory card at $80. Still restarting.
When we got involved, the diagnostic buffer showed a cycle time violation. The watchdog was tripping because a data block in user memory had gotten corrupted. We reloaded the program from a verified backup and the issue disappeared. Cost of the fix itself: minimal. But by then, the customer had spent $195 on unwanted parts, lost two and a half days of production, and paid for two emergency call-outs. That "$115 fix" ended up costing north of $9,000 when downtime was factored in.
I'm not saying replacement parts are bad. They're an essential part of maintenance. But I firmly believe that total value matters more than unit price. A $500 diagnostic visit that solves the problem in one day is cheap. A $115 part that doesn't fix anything is expensive — even before you count production losses.
Here's the total cost equation that too many people skip:
- The part itself
- Shipping and procurement time
- Installation labor
- Production downtime during each attempted fix
- Emergency service premiums when you escalate
- The hidden cost of repeated failures and eroded confidence
Add those up, and systematic diagnosis is almost always the lower-cost path. The "cheap fix" is usually an expensive gamble.
Reliability: The Three-Week Curse
The third difference between these approaches is about what happens a few weeks after the fix.
Parts-swapping tends to produce temporary relief. That's not a coincidence — when you replace a marginal component, the system might stabilize for a while. But if you haven't addressed the root cause, the condition that created the original failure is still there. The restarts come back.
I call it the "three-week curse." In 2022, a food processing plant lived with this cycle for six months. They replaced the battery, swapped the power supply, reseated every card. Restarts would stop for two or three weeks, then return like clockwork. When we finally traced the issue to a deteriorating backplane connector, they spent $900 on a rack replacement. The problem never came back.
That's the core reliability difference: the systematic approach targets the actual failure mechanism, which gives the fix a realistic chance of being permanent. The parts-swap approach is a series of temporary patches that can go on for months.
Comparison at a Glance
| Dimension | Systematic Diagnosis | Parts Swapping |
|---|---|---|
| Diagnostic accuracy | High — uses the PLC's own diagnostic tools | Low — root cause rarely identified on the first try |
| Cost profile | Higher upfront, typically lower overall | Lower upfront, but repeat failures add up fast |
| Downtime | Usually 1–2 site visits | Often 3–5+ visits before resolution |
| Fix durability | Permanent when root cause is addressed | Often temporary — the "three-week curse" |
| Risk | Low, methodical | Moderate — ESD damage and connector wear from repeated swaps |
What I'd Recommend
Honestly, I'd rarely recommend starting with parts swapping. But there are scenarios where it makes sense:
Start with systematic diagnosis when:
- The restarts are intermittent — the classic S5 restart running problem
- You have access to STEP 5 or the diagnostic buffer
- Production downtime is expensive (let's be honest, it always is)
- You have a verified backup of the user program
- The S5 is mission-critical and needs a durable fix
Parts-swapping can be justified when:
- You already have identical spare parts on hand (zero incremental cost)
- The S5 is scheduled for retirement within weeks anyway
- You've already done a diagnostic pass and identified a likely suspect
- You need a quick intervention to get production running right now
But set a strict budget: two component swaps maximum. If the restarts continue after that, stop. Run the diagnostics. I've seen too many plants throw $3,000 into chasing a "$115 problem" because of the sunk cost fallacy.
I'll also be straight about my mixed feelings on the bigger question. Part of me wants to tell every S5 owner to migrate to a modern platform like the S7-1200 or S7-1500. Another part knows these old systems can run reliably for years with proper diagnosis and maintenance. The right answer depends on your spares availability, your production risk, and your budget — not on what's fashionable.
Bottom Line
The Siemens S5 restart running problem is a textbook case of the cheapest-looking fix being the most expensive one. I still kick myself for the early years when I approved parts-swap recommendations without pushing for diagnostics first. If I'd asked for the interrupt stack data on the first visit, we'd have saved customers tens of thousands of dollars in unnecessary replacements and downtime.
The S5 is telling you what's wrong. The diagnostic buffer and interrupt stack were designed for exactly this situation. Take the time to read them.
And honestly, this principle goes beyond Siemens. Whether you're debugging a Siemens PLC, a Rockwell VFD, or any other industrial controller, the rule is the same: diagnose first, replace parts second. Your production line — and your maintenance budget — will thank you.
Prices mentioned are as of early 2025; verify current rates. Diagnostic procedures should be performed by qualified personnel following Siemens' official documentation.
