Supplier quality audits: what a day inside their plant can and cannot tell you

Go where your part is, not where the tour goes

The most productive structure is to pick one of your own components and trace it end to end on their floor: incoming material and its certification, the storage condition, the setup and its approval, the in-process checks and what happens when one fails, the containment of suspect stock, final verification, packing and labelling. At each step ask for the record of the last time it happened, not a description of how it should. This exposes breaks in the chain that a departmental tour smooths over, and it keeps the audit anchored to something you can act on commercially.

The signals that read faster than any document

Experienced auditors form a view within the first walk. Is nonconforming material physically segregated and labelled, or in an unmarked pallet at the end of an aisle. Are containers identified with a lot reference or only with a part number. Do gauges at the line carry current calibration status. Is there a visible reaction plan at the point of use, and can the operator explain what they do when a reading goes out of limit. Are there several revisions of the same drawing in circulation. None of these prove a defect, but a cluster of them predicts one reliably.

Announced, unannounced, or a return visit

Announcing gives you the right people and records and costs you the working state of the plant. An unannounced visit shows the plant as it runs but can waste the trip if the process is idle or the engineer is away, and it spends goodwill you may need later. A practical pattern is to announce the initial audit, then use short unannounced or short-notice returns when a specific concern exists — after an escape, after a reported process change, or when a previous finding was closed on paper. The return visit is where most credibility is won or lost.

Writing findings a supplier can actually act on

A finding needs the evidence attached: the machine, the part, the record examined, the date it was generated, and the requirement it fails against. Vague criticism invites a defensive response and an action plan of generalities. Separate what breaches an agreed requirement from what is simply not how you would run it, because mixing the two lets the supplier dispute the whole report. Agree the findings verbally before leaving, with the people who will own them present, since a report that arrives days later to an unprepared reader loses most of its force.

The limits of an audit day, and what to pair it with

An audit samples a shift under observation at a site that knew you were coming. It cannot tell you how the night shift behaves, whether the process holds through a tooling change, or what happens under a demand spike from a larger customer. So weight it alongside evidence generated over time: your own incoming rejection history, escape events, responsiveness on corrective actions, and whether they tell you about problems before you find them. A supplier who reports their own escape is usually safer than one with an immaculate audit score.

Frequently asked questions

Is a third-party certificate a substitute for auditing a supplier ourselves?

It substitutes for auditing their management arrangements, and not for looking at your part. Certification assesses a scope defined by the supplier against a general framework; it does not examine whether your specific component is made on capable equipment with the checks you require. Sensible practice is to accept the certificate as coverage of the system layer, then spend your own visit on the process making your parts, which is where your risk actually sits.

Can we audit a sub-supplier two tiers down?

Only if your contract gives you access or your direct supplier arranges it, and even then you are a guest with limited standing. Where the deeper tier carries the real risk — a special process, a single-source material, a coating house — the practical routes are to require your supplier to demonstrate their own control of that source, to specify the sub-supplier yourself, or to join their audit rather than mount your own. Attempting it without agreement damages the relationship you rely on.

How do we audit a supplier we cannot travel to?

Remote sessions work for records, procedures, training evidence and interviewing process owners, and they are useful for follow-up on known findings. What they cannot do is show you segregation, housekeeping, labelling discipline, or the material you were not invited to see, because the camera goes where the host points it. Where travel is impractical for a high-risk part, a competent local second party doing the physical walk is usually a better answer than a video call.