6 Things to Know Before You Buy Obsolete PLC Parts (Because It Costs More Than You Think)

Hunting for obsolete PLC parts or a drive? Here’s how to avoid fakes, dodge long lead times, and get your line running again faster.

When a controller fails on your line, and the manufacturer stopped making it years ago, the clock starts ticking. Manufacturing is a major part of the economy (in the UK alone, it makes up around 10% of GDP and supports roughly 2.7 million jobs [1]), and every hour of downtime in it costs money. The search for a replacement part can feel like a maze of listings, brokers, and “in stock” PLC part claims that don’t always hold up.

Whether you’re sourcing obsolete PLC parts, a discontinued drive, or a surplus I/O module, a few checks before you hit “buy” can save you a second failure a few weeks down the line. Here’s what actually matters.

1. The real cost is the wait, not the PLC parts

The price tag on an obsolete part is rarely the biggest number in this equation. Lost production while you wait for a quote, a shipment, or a supplier who “should have it by Friday” usually costs far more than the part itself.

Unplanned downtime is now estimated to cost the world’s largest manufacturers around $1.4 trillion a year between them, roughly 11% of their revenue [2]. A separate global survey of plant maintenance leaders found the typical industrial business loses about $125,000 for every hour of downtime [3]. However you size your own operation, the gap between “wait a week for the cheapest listing” and “pay more for stock that ships today” is usually bigger than it looks on paper.

Before you buy, ask: what does one hour of downtime actually cost my operation? That number should shape how much you’re willing to pay for speed, not just for the part.

2. Know whether it’s new, surplus, or refurbished

These terms get used loosely, and not every seller explains the difference:

• New surplus: unused stock, often from decommissioned lines or excess inventory
• Refurbished: used, then inspected, cleaned, and tested
• Used / as-is: sold without testing or guarantees

None of these are automatically bad, but the price should match the condition. If a listing doesn’t say which one you’re getting, ask before you pay.

3. Match the full part number, not just the model of the PLC Parts

Many legacy parts, particularly older ABB, Siemens, and Allen-Bradley ranges, look identical across revisions but aren’t interchangeable. Two modules can share a model number and still differ in firmware version, hardware revision, or series, and that difference can be enough to stop the part from working in your system.

We’ve seen this with older ABB Advant and S800 hardware, where two units with near-identical labels turned out to be entirely different revisions. The fix is simple: compare the full label on the part in your machine against the listing, down to the revision letter, and send a photo to the supplier if you’re not sure.

4. Watch for red flags on counterfeit or untested stock

A few warning signs are worth taking seriously:

• Prices well below every other listing for the same part
• Stock photos instead of real photos of the actual unit
• No mention of testing, condition, or warranty
• Vague or shifting answers about where the PLC parts came from

None of these guarantees a problem on its own, but two or three together are a reason to ask more questions before you order.

5. Ask about testing and warranty

A reputable supplier should be able to tell you how a part was tested and what happens if it doesn’t work once it’s installed. Even a short warranty period is a good sign that the seller stands behind what they’re shipping, rather than moving stock and hoping for the best.

6. Consider repair as a backup plan

If the exact PLC parts you need are out of stock everywhere, repair can sometimes get your line running sooner than continuing the hunt. It’s worth asking whether the faulty unit can be repaired in parallel while you look for a replacement, especially for PLC parts with long or unpredictable lead times.

Quick checklist before you buy

• Do I have the full part number, revision, and series confirmed?
• Is it listed as new, surplus, or refurbished, and does the price reflect that?
• Was it tested, and is there a warranty if it fails?
• What’s the realistic delivery time, not just the listed one?
• Should I keep a spare on hand for next time, given the lead time on this part?

FAQ’s

How long do obsolete PLC parts usually take to source? It varies widely by manufacturer and part, from a few days for common modules to several weeks for rare or heavily discontinued ones. Suppliers with existing global stock networks can usually move faster than a cold search.

Is it safe to buy refurbished PLC parts or drives? Yes, provided it’s been properly tested and comes with some form of warranty. Ask the supplier what their testing process covers before you buy.

What’s the difference between obsolete and legacy equipment? “Legacy” generally means older equipment that’s still supported to some degree. “Obsolete” means the manufacturer no longer produces or officially supports it, which is when sourcing gets harder and specialist suppliers matter more.

Need a hard-to-find part? At Mizen Automation, we specialise in PLC repair and sourcing obsolete industrial PLC parts, including legacy ABB, Siemens, and Allen-Bradley hardware. Send us the part number, and we’ll help you track it down.

References

[1] Office for National Statistics / House of Commons Library, UK manufacturing sector data

2] Siemens, The True Cost of Downtime 2023

[3] ABB, Value of Reliability: ABB Survey Report 2023