Buying Automation Spares? Your Situation Matters More Than the Part Number

When I took over purchasing for our plant in 2020, I treated every automation parts request the same: find the part number, get the best price, and hope accounting doesn't kick the invoice back. That worked for about six months.

Then I realized something. The right way to buy an industrial automation spare isn't determined by the part itself. It's determined by your situation.

In my five years handling procurement for our plant—roughly $120K in annual orders across eight vendors, mostly ABB VFD controllers, I/O modules, and the parts that support them—I keep running into three distinct situations. Each one needs a different strategy:

• The emergency: production is stopped or about to stop. The part has to ship today.
• The stocking order: you're planning ahead. Hopefully the part just sits on a shelf.
• The obsolescence call: the part you used to buy doesn't exist anymore. Now you're choosing between patching and migrating.

I've bought digital output modules, processor module CPUs, analog modules, and various sensors in all three scenarios. Here are the patterns I've found.

Scenario 1: The Machine Is Down (Emergency Replacement)

You know you're in this scenario when the plant manager is standing at your desk asking how fast the part can get here. The budget question disappears. Speed is the only question.

But here's the trap: speed doesn't mean skipping verification. The worst outcome in an emergency isn't the wait—it's getting the wrong part, fast. A digital output module can look identical between drive generations but have a different pinout or firmware. Confirm the exact part number, revision, and compatibility before you click order. ABB's type code documentation (at new.abb.com) makes this straightforward, but it only helps if you actually check it. (note to self: this has saved me at least three times in 2024 alone.)

I learned the verification lesson the expensive way. In 2022, a supplier I'd never used offered a "compatible" digital output module for $160 less than the genuine part. He had a website, a phone number, and a convincing pitch. I ordered it. It failed within an hour of installation. The drive faulted, the line stopped, and we had to buy the original part anyway, plus rush shipping. That $160 "savings" turned into roughly $3,800 in downtime and shipping (ugh).

The conventional advice: don't experiment with gray-market parts in an emergency. It's true.

But don't assume going direct to the manufacturer is your fastest option either. In my experience, an authorized distributor with stock in hand can ship faster than a factory order—they check the part, confirm compatibility, and print the paperwork while you're still on the phone. When the line is down, that matters more than the price.

My emergency checklist, in order: part number confirmed, revision matched, vendor's test report requested, shipping method confirmed by phone.

Scenario 2: Building a Spares Stock (Proactive)

The second scenario arrives without a fire. Maybe you had a close call, maybe an auditor made a recommendation, maybe you just want to sleep better at night. You're buying a part destined for a shelf.

This is where most buyers, including me, make the biggest mistakes.

Stock by criticality, not by price. Everyone stocks the cheap items—fuses, relays, standard digital input and digital output modules. But nobody wants to allocate budget for a $2,000 processor module CPU. Which one, honestly, will stop the line when it fails? Cheap parts can be sourced quickly. The expensive critical components are the ones with three-week lead times. Those are the ones worth stocking.

Treat analog spares differently from digital spares. Digital I/O modules are fairly standardized—they either work or they don't. An analog module PLC spare, though, can drift out of calibration while sitting on a shelf for two years. If you stock analog modules, build a testing schedule or a rotation plan, not just a purchase order.

Same logic applies to sensors. An analog distance sensor is usually tied to a specific machine or application. Unless you have multiple identical stations using it, stocking one is usually a waste—it'll go obsolete before it gets installed. Better approach: document the lead time and keep a vetted vendor ready.

And test your spares. A spare you've never tested is imaginary. When the processor module CPU arrives, bench-test it, then schedule a re-test every year. It's tedious. It's also the difference between a spare and a paperweight. (I really should schedule the annual test for the CPU we stocked in January—noted.)

Honesty moment: even after we approved the budget for that spare processor module CPU, I spent two weeks second-guessing. What if we should've bought a lower-cost alternative? What if the revision gets superseded next year? It's been in the cabinet for eight months now, and the testing plan is still undone. The purchase decision was the easy part.

Scenario 3: The Part Is Discontinued (Migration Time)

The third scenario sneaks up on you. A supplier says "no longer available." Lead time jumps to 14 weeks. Or the only stock left costs triple and it's labeled a "last-time buy."

For us, it was a panelview 700 HMI on an older packaging line. We'd replaced the screen twice already, and we all knew the next failure was a matter of time.

The classic mistake is to keep patching an obsolete platform with used parts. It feels responsible. It feels like saving money. But every dollar spent keeping a discontinued product alive is a dollar that should be going toward migration.

Here's the decision frame I use:

1. How much life is left in the machine? If the line gets replaced in two years, buy the refurbished spare and move on. Just don't let the stopgap become permanent. Set a reminder to reassess in 12 months.
2. If the machine has five or more years of life left, the migration conversation should have started yesterday. A $300 used HMI keeps you running today, but the third $300 used HMI is a down payment on a project you're avoiding. Don't let a small part make a big strategic decision.
3. If you do a last-time buy, demand documentation. A refurbished legacy module with a test report is worth three times the same module without one. I once bought a discontinued module from a broker who couldn't produce a proper invoice. Finance rejected the expense; the module went back; the loss came out of our department budget. Don't hold me to this, but I've seen last-time-buy prices run 50-100% above the original list price—documentation is the only leverage you have.

The uncomfortable truth about obsolete parts: denial is the real problem. When a part becomes unavailable, that's a signal to start a conversation, not to go hunting for the next used one.

Which Situation Are You In? Three Questions

Here's the framework I use when a request lands on my desk:

1. Is production stopped or at risk within the next 48 hours? If yes, you're in Scenario 1. Execute the emergency checklist. Don't get distracted by sourcing experiments.
2. Is this part going on a shelf for later? If yes, you're in Scenario 2. Ask whether it's truly critical, and set up a testing plan before you file the PO.
3. Did the manufacturer say "no longer available"? If yes, you're in Scenario 3. Even if you're fixing today's problem, schedule the migration conversation now.

These situations overlap more than you'd think. When they do, handle the emergency first—but then make a deliberate choice about the next step. The biggest mistake I've made as a buyer is solving the same crisis three times instead of stopping to ask why it keeps happening.

The Bottom Line

The most expensive part you can buy is the one that doesn't work when you need it. The second most expensive is the one you bought without asking which scenario you're in.

I keep this three-scenario note stuck to my monitor. It has saved me more money than any pricing spreadsheet I've ever built. If you're not sure which situation applies to you, start with the three questions above—and be honest with the answers.

That's it. Three situations, three questions, one honest answer.

Pricing and availability information is for general reference only, based on procurement experience through Q1 2025. Verify current pricing, part numbers, and compatibility with an authorized distributor before placing orders. Product names referenced belong to their respective manufacturers.