Sourcing · 9 min read · Updated November 2026
Buying Surplus and Refurbished PLC Parts Without Getting Burned
When the manufacturer has stopped making it, the surplus market is the only market. It is also where relabelled, repaired and counterfeit parts circulate under the word "new". The checks that separate a usable spare from an expensive paperweight, and the questions to ask any seller before paying.
The surplus market for automation parts has four tiers, and the seller rarely tells you which one you are in. New surplus is unused product in original packaging from distributor overstock, cancelled projects or end-of-line purchases — genuinely new, sometimes years old. Pulled / used is product removed from a working machine, functional but with history. Refurbished is a used or failed unit that has been repaired, cleaned and tested, ideally with a report. Counterfeit or relabelled is anything whose label does not tell the truth: a repaired unit sold as new, a lower-spec unit relabelled as a higher one, or an outright copy. All four tiers appear on the same marketplaces at similar prices. Here is how we sort them, both when we buy and when we sell.
First principle: the label is evidence, not proof
Every manufacturer encodes useful information on the product label. Siemens prints the order number, a product version (e.g. "FS: 03"), firmware level where relevant and a serial number; from 2020 most labels carry a QR code that resolves on Siemens Industry Online Support to the product and its manufacturing date. Rockwell prints the catalog number, series letter and a date code; Rockwell's "Product Compatibility and Download Center" identifies firmware families. Mitsubishi prints a serial with the manufacturing year and month encoded. Omron and Schneider use lot numbers. Check three things: the label matches the housing (fonts, alignment, laser marking versus printed sticker), the serial resolves in the manufacturer's tool or matches the format for that year, and the manufacturing date is plausible for the claimed condition — a "new" S7-300 CPU with a 2012 date is new surplus, which is fine if priced as such, but a "new" unit with a 2024 date for a product cancelled in 2025 deserves a question, because very few were built that late.
The physical inspection
- Packaging. Original box with matching label, anti-static bag sealed, foam inserts present. A new CPU in a plain carton is used until proven otherwise.
- Connectors and terminals. Screw heads with tool marks, tinned wire fragments in terminals, scuffed DIN-rail clips, dust in ventilation slots — signs of previous installation. Not disqualifying for a used part, disqualifying for one sold as new.
- Housing. Replaced housings are common on refurbished units: look for mismatched plastic shade between front and back, missing or re-glued labels, filled screw holes.
- Memory cards and batteries. Batteries in used units are usually flat; a seller who includes a fresh battery and says so is a good sign. Memory cards may hold a previous owner's program — wipe before use.
- Repair traces. Flux residue around components, replaced electrolytic capacitors with different brands, or hand-soldered leads indicate repair. Legitimate refurbishers document this; relabellers hide it.
Testing: what a credible test report contains
"Tested working" means nothing without the method. For a PLC CPU we expect, and we provide: power-up on a reference rack with the correct power supply; firmware version read out and recorded; a test program downloaded, run and read back; communication on every port (MPI/DP, PROFINET, Ethernet, serial) verified against a partner device; battery/retentive memory check; and, for CPUs with integrated I/O, every input and output exercised. For I/O modules: every channel, both states (or analog at 0 %, 50 %, 100 % with a calibrator). For HMIs: backlight hours if readable, touch calibration across the whole screen, every port. For drives: powered run on a motor at rated current for at least 30 minutes, with the parameter list reset to defaults. The report should name the person, the date and the serial number — ours does, with photographs.
Firmware and revision compatibility
The most common reason a surplus part fails to work is not that it is faulty but that it is the wrong revision. Rules of thumb:
| Brand | What to match | Typical problem |
|---|---|---|
| Siemens S7-300/400 | Order number incl. last digits, firmware version family | Project compiled for V3.x fails on V2.x CPU; MMC size and type |
| Siemens S7-1200/1500 | Order number suffix (-0XB0 vs -0XB5), firmware version, generation (G1/G2) | TIA project targets a firmware newer than the spare; G1/G2 modules not interchangeable |
| Allen-Bradley SLC/MicroLogix | Catalog number, series letter, OS/firmware (e.g. OS501 FRN 11) | RSLogix 500 project revision mismatch; Series A/B/C I/O behaviour |
| Allen-Bradley Logix | Catalog number, series, firmware major revision | Studio 5000 project at revision 33 cannot download to a controller at 20 without flashing |
| Mitsubishi Q/FX | Model, serial (first 5 digits = version), function version | Instructions unsupported on older serial; GX Works3 features by serial |
| Omron CJ/CS | Model, unit version (Ver. 1.x–4.x), region suffix | CX-Programmer project uses instructions above the unit version |
| Drives (ABB, Danfoss, Siemens) | Type code incl. options, firmware, option boards | Parameter file from newer firmware will not load; missing EMC filter or brake variant |
Always send the seller the exact order number and the firmware your project needs, and ask them to confirm both in writing before shipping.
Pricing logic
Surplus pricing follows scarcity, not list price. A CPU 315-2 PN/DP sells well above its last list price; an SM 321 digital input sells below. Refurbished should be 40–60 % of new surplus; used/pulled 25–50 %. If a seller's "new" price is below the refurbished price of reputable houses, it is probably not new. Warranty is the other signal: reputable surplus suppliers give 12 months on new surplus and 6–12 months on refurbished, and will take a dead-on-arrival unit back without argument. A 30-day warranty on a "new" PLC is a used PLC.
Questions to ask before paying
- Is the unit new surplus, used or refurbished? What is the manufacturing date on the label?
- Can you send a photograph of the label and the serial number before shipping?
- What firmware/series/version is it, and can you confirm it supports my project revision?
- What test did you perform, and will you send the report with the unit?
- What is the warranty period and return policy for DOA?
- Where did the unit come from (distributor stock, plant decommissioning, repair)?
A seller who answers all six quickly is the seller to keep. We answer them in the quotation before you ask, because the alternative — a line down for a week with a dead "new" spare — costs everyone more than the discount was worth.
Need engineering help with this?
Our engineers apply this to real projects every week — selection, specification, commissioning. Send your question or a part number.
Quick answers
Is it legal to sell used or surplus PLC parts?
Yes. The issue is description, not legality: a unit must be sold as what it is. Selling a repaired or used unit as new is fraud in most jurisdictions and voids the manufacturer warranty.
Does Siemens or Rockwell honour warranty on surplus parts?
Generally no — manufacturer warranty runs from the original sale through an authorised channel. The surplus supplier's own warranty applies, which is why its length and terms are a quality signal.
Do you sell refurbished parts?
Yes, clearly labelled as such, with the test report, the repair description where applicable and a 6–12-month warranty. New surplus and tested used units are labelled the same way. We never describe a repaired or used unit as new.