A supplier audit should reveal how a factory actually controls your product, not merely whether it has certificates and written procedures. The most useful audits follow a real order from incoming material through production, inspection, release and shipment. They combine document review with observation, interviews and verifiable records.
This guide explains how to conduct a supplier audit that supports a sound sourcing decision and improves ongoing control. For a ready-made list of audit topics, use our manufacturing supplier audit checklist.
1. Define the purpose and risk-based scope
Start by deciding what the audit must answer. A pre-qualification audit checks whether a new factory can manufacture the product. A process audit examines a specific production route. A corrective-action audit verifies whether earlier problems have been permanently resolved. The scope should reflect product risk, technical complexity, annual spend, regulatory exposure and the consequences of supply interruption.
A broad, generic audit can miss the controls that matter most. For a machined safety-critical component, focus on material identity, programming, tool control, measurement and traceability. For a coated steel assembly, include surface preparation, bath or paint parameters, adhesion testing and corrosion requirements.
2. Prepare using product-specific evidence
Before visiting, review the drawing, specifications, process flow, previous quotations, samples, complaints and delivery performance. Ask the supplier for relevant certificates, organisation chart, equipment list and subcontractor overview. Identify the critical product and process characteristics you want to trace on the shop floor.
Create an audit plan with timing, participants, departments and required records. Share the agenda, but keep enough flexibility to follow evidence when it points elsewhere. If this is a new source, combine the audit with the broader steps used to qualify manufacturing suppliers.
3. Open with clear expectations
During the opening meeting, confirm the scope, confidentiality rules, safety requirements and expected access to records. Ask management to explain the site, products, processes and current constraints. Keep the introduction concise; the main value comes from observing operations and checking evidence.
4. Follow a real order through the factory
Select a current or recently completed order that resembles your product. Trace it backwards and forwards through the process. Check how the supplier:
- verifies the contract, drawing revision and technical requirements;
- approves incoming material and records certificates or lot numbers;
- releases work instructions, programs and inspection plans;
- identifies work in progress and segregates nonconforming parts;
- controls outsourced heat treatment, coating or testing;
- performs final inspection and authorises shipment;
- retains records and links them to the delivered batch.
This transaction-based approach exposes gaps between procedures and everyday practice. It also shows whether manufacturing supply chain traceability works under normal production conditions.
5. Interview operators and inspect evidence
Ask open questions such as “How do you know which revision to use?” or “What happens when this measurement fails?” Operators should be able to explain their task, the acceptance criteria and the escalation route. Compare their answers with posted instructions, system data and completed records.
Do not rely only on clean presentation rooms or selected samples. Check calibration labels, maintenance status, material segregation, first-off records, nonconformance areas and actual measurement results. Photographs can support the report when permitted, but they do not replace notes that identify the order, process and record reviewed.
6. Assess subcontracted processes and supply risk
A factory may outsource coating, heat treatment, special testing or even complete production steps. Verify how those subcontractors are approved, monitored and linked to the product records. Ask what happens if a key machine, tool or sub-supplier becomes unavailable. Our guide to supplier risk management in manufacturing provides a broader framework for continuity and dependency risks.
7. Classify findings clearly
Each finding should state the requirement, objective evidence and observed gap. Avoid vague comments such as “quality control needs improvement”. A useful finding identifies exactly what was missing or ineffective and why it matters.
Classify findings consistently, for example as critical, major, minor or opportunity for improvement. The definitions should be agreed before the audit. A critical issue may require an immediate sourcing hold, while a minor issue can often be closed through a documented correction.
8. Agree corrective actions and verify effectiveness
The closing meeting should summarise strengths, findings, immediate containment and deadlines. Corrective action must address root cause, not only replace a defective part or rewrite a procedure. Require an owner, due date and evidence for every action.
Closure is not complete when the supplier sends a document. Verify that the action has been implemented and is effective through records, remote evidence, new samples or a follow-up visit. Include ongoing controls in the supplier quality and inspection plan.
Conclusion
An effective supplier audit is focused, evidence-based and connected to the actual product. It tests whether people, processes and records consistently deliver the required result and whether weaknesses are resolved. TOS can support supplier qualification, on-site audits, corrective-action follow-up and European quality expectations. Contact TOS Outsourcing to plan an audit or supplier assessment.