The Supplier Quality Checklist: What I Audit Before Approving Automotive Parts

Who this checklist is for

If you're the person who has to approve automotive stampings, CNC machined parts, forged parts, or aluminum extrusions, this checklist is for you. I work at Optimal as a quality and brand compliance manager. I review parts and process records before they ship, and I've seen enough near-misses to know that a supplier's sales deck and their production floor are not always the same story.

In Q1 2025, I rejected about 6% of first-article submissions. That number might sound high if you haven't been inside a machine shop. We're not trying to be difficult. We're trying to keep a bad part from leaving the building and taking someone's brand reputation with it.

This isn't a full PPAP audit. It's a practical pre-screen—the checklist I run through before a new supplier earns a place on our approved vendor list.

Step 1: Start with the part family, not just the print

Every part family has a different quality risk. A stamped bracket can have springback. A forged steering arm can form laps or seams. An aluminum extrusion for a structural rail can have porosity or crush issues. If the supplier can't tell you what tends to go wrong in their own process, you've just found the first red flag.

Ask them to walk through the process flow at production volume: press speed, heat treat, deburring, machining, surface treatment, inspection. The part may be an automotive bracket today, a marine fuel transfer pump housing tomorrow—the critical question is always whether the supplier owns the operation that makes or breaks the part. That answer tells me where I should spend my audit time.

Step 2: Ask who builds and fixes the dies and molds

For stampings, extrusions, and forgings, the tool is the product. I'm less interested in whether a supplier has a toolroom than in how they control tooling changes.

Ask for the last three tooling maintenance records. Look for dates, reasons, and what was inspected after the tool changed. A die can make perfect parts before a repair, then produce 2,000 bad pieces after a subtle modification if nobody validates the change. If the supplier can't show you that trail, the tooling might not be as controlled as they think.

At Optimal, custom die and mold manufacturing is part of our workflow, so tooling modifications go through the same review as production parts. But I've also audited suppliers where the die was modified by an outside shop and no one on the production floor knew. That's how the part that 'passed last month' fails this month.

Step 3: Define the measurement method before you quote

This is the step most people skip because it sounds like paperwork. Then it becomes an argument later.

A drawing says a hole diameter is 10.0 +/- 0.1. If the inspector measures with calipers over a burr, they might get 9.85. If another inspector uses an air gauge on a deburred part, they get 10.02. The part hasn't changed; the measurement system has.

So put measurement requirements in the RFQ: the gauge type, the temperature condition if it matters, the calibration standard, and how surface texture will be judged. The AIAG PPAP 4th edition has been the common automotive reference since 2006, and as of 2025 it's still in most customer contracts. But a generic reference is not enough. Be specific about critical dimensions.

If I could redo one earlier supplier qualification, I'd have required them to share their CMM measurement program before first article. I assumed all CMMs would calculate a diameter the same way. They don't. That assumption cost us three weeks and a re-qualification.

Step 4: Demand process evidence, not a single Cpk number

A Cpk of 1.67 from five parts doesn't prove a process is capable. It proves someone picked five good parts. If the process is drifting, the tenth hour of a run will be worse than the first.

Ask for a run chart, histogram, or averages and ranges from a production segment. If a dimension is critical, I want to see stability across the run. When you do this, you'll also find suppliers who don't track anything except final scrap. That's useful information.

IATF 16949:2016 is the automotive quality management standard that pushes suppliers to monitor process capability instead of just checking parts after the fact. As of 2025, any supplier with automotive experience should recognize that language. If they only want to talk about certificates, keep looking.

Step 5: Sample from a real production run, not a handmade first article

First articles are often handled more carefully than production parts. A skilled toolmaker can deburr, polish, and adjust a sample until it looks perfect. That tells you the tool can make the part. It doesn't tell you the process can make it consistently at speed.

Ask for a trial run at normal production conditions. Take samples at the start, middle, and end. Check whether the dimension drifts as tooling heats up or as material coils change. If a supplier won't do this, or wants extra money for it, I still recommend paying. It's cheaper than a sorting line later.

I remember a die that looked fine in the toolroom. On the press, at 40 strokes per minute, the material started to bounce and we found micro-cracks at the bend radius. We caught it because we waited for a real run. The supplier was embarrassed, but the issue cost a few hours instead of 8,000 bad parts.

Step 6: Ask how they handle defects before they have yours

No plant is defect-free. What separates a good supplier is what happens after a nonconformance.

Ask for a recent corrective action report, redacted if needed. Ask who decides whether suspect inventory gets contained. Ask if they sort in-house or wait until the defect shows up at your dock. A supplier that can tell you about a real failure and the fix they put in place is more trustworthy than one who says 'zero defects, always.'

To be fair, not every small machine shop has a formal 8D process. But they should be able to show you a scrap log, a repair log, or some way of learning from mistakes. If they can't name anything that changed because of a previous problem, your problem will probably be the first one.

Step 7: Think about your customer's perception, not just the print

Let me give you an example. A stamped aluminum cover was functionally fine, but it had visible surface waves. It would have sealed correctly, fit correctly, and probably lasted as long as the vehicle. I rejected it anyway, because the part was a visual feature on a premium product. Customers don't buy a car and think 'that wave is within drawing tolerance.' They think 'this brand uses cheap parts.'

The redo cost about $22,000 and delayed a launch by a week. Some people thought I was being unreasonable. But that $22,000 protected a much bigger relationship. This is what I mean when I say quality is brand image.

The same applies to hidden parts. If an oil pan leaks or keeps a transmission from reaching optimal transmission temperature, no customer will see the pan. They'll just experience the failure. Quality perception is built on things that work—not only on things that look good.

Three mistakes to avoid when you use this checklist

Mistake 1: Treating supplier qualification as a one-time event. Quality changes with every tooling repair, every process move, and every new operator. I re-check after the first production order, after a die modification, and after the supplier switches material vendors.

Mistake 2: Waiting until final inspection to find a problem. If a process is unstable, final inspection only tells you that you now own a bad lot. The earlier you sample, the less expensive the lesson.

Mistake 3: Using quality issues to push for lower prices. When a supplier realizes that every problem becomes a discount negotiation, they'll stop reporting defects. Defects don't disappear when you stop talking about them. They just show up in your customer's hands.

At the end of the day, 'optimal' isn't a slogan. It's the output of a process that measures, controls, and reacts. Use this checklist and you'll protect the supplier, the part, and—most importantly—your brand.