A supplier questionnaire tells you what a manufacturer says about themselves. An on-site audit tells you what is actually happening. The gap between these two things is the reason that facility audits remain the most reliable supplier qualification tool available, regardless of how sophisticated the document review process is.

This guide covers how to plan, conduct, and document a manufacturer audit — not from the perspective of a compliance function running against a checklist, but from the perspective of a procurement team that needs to make a qualified sourcing decision.

When to Conduct a Manufacturer Audit

Not every supplier relationship requires an on-site audit. A risk-based approach to audit planning focuses investment where exposure is highest:

New supplier qualification. For any new supplier representing significant spend, single-source risk, or supplying safety-critical components, an on-site audit before placing the first production order is standard practice.

Cause-based audits. When quality performance deteriorates significantly, when a supplier reports a major process change or facility move, or when a serious quality escape occurs, a for-cause audit is the appropriate response.

Periodic surveillance audits. For approved suppliers, periodic surveillance audits (typically every 2–3 years, more frequently for critical suppliers) verify that the supplier’s processes and quality system have not degraded since qualification.

Remote audit considerations. For low-risk, low-spend suppliers where travel is not economically justified, document review and video walk-through audits provide reduced coverage. These are supplements to, not replacements for, on-site audits when the stakes are high.

Pre-Audit Preparation

An audit is most valuable when you arrive knowing what you are looking for. Pre-audit preparation:

Define the audit scope. What specific processes, quality systems, and capabilities are you auditing? The scope should be driven by the specific products you intend to source. An audit of an electronics assembly supplier focuses on different process areas than an audit of a precision machining shop.

Review documentation in advance. Request and review the supplier’s quality manual, process flow diagrams for the relevant product category, and any prior audit reports (internal, customer, or registrar audits) before the visit. Understanding their documented system before you arrive lets you evaluate the gap between documentation and practice rather than spending audit time learning the documented system.

Develop an audit checklist. Build a checklist specific to the product and process type — not a generic checklist that misses the technical risks specific to your supply requirement. For an ISO 9001-certified supplier, the certification standard provides a starting framework; add process-specific technical questions.

Identify the right audit team. A good audit team includes at least one person with technical expertise in the relevant manufacturing process and one person with quality management system expertise. A procurement professional alone cannot evaluate SPC implementation or process capability — bring the right people.

What to Assess During the Audit

Facility and Equipment Condition

First impressions are data. The condition of the facility — cleanliness, organization, equipment maintenance — correlates with operational discipline. A manufacturing floor where material is properly labeled and stored, work instructions are visible at workstations, and equipment is visibly maintained signals process discipline. A chaotic, poorly organized facility is a risk signal regardless of what the quality manual says.

Specific equipment observations:

  • Is inspection equipment calibrated and in current calibration status? (Check calibration tags on gauges and measurement tools)
  • Is production equipment maintained? (Look for maintenance logs at equipment, observable equipment condition)
  • Is the facility organized to prevent commingling of conforming and nonconforming material?
  • Is temperature, humidity, or cleanliness controlled appropriately for the product type?

Quality System Implementation

The audit objective is to verify that the documented quality system is actually being followed — not just that documentation exists.

Work instructions: Are written work instructions available at workstations? Do they reflect current practice, or are they outdated? Ask an operator to walk you through how they use the work instructions.

Statistical process control: If the supplier claims SPC implementation, look for control charts at production stations. Ask to see the most recent charts for a critical characteristic. Are the charts actively maintained, or are they partially filled and stuck to the wall as decoration?

Nonconformance handling: How are nonconforming parts physically segregated and marked? Ask to see the nonconformance log and a recent corrective action record. The quality of their corrective actions reveals the quality of their quality system.

Calibration system: Is there a master list of calibrated instruments? Are calibration records current? Who is responsible for the calibration program?

Training records: Are operators trained and do training records exist? Ask to see the training record for an operator you observe — specifically, can they demonstrate that the person running a specific process is qualified to do so?

Process Capability and Production Technology

Verify that the supplier has the equipment and process capability for the specific tolerances and specifications in your parts:

  • Do they have the right equipment for the process (correct machine type, adequate capacity)?
  • For precision work, what are their demonstrated process capabilities (Cpk) on similar parts?
  • For special processes (heat treatment, plating, welding), are they performed in-house or sub-contracted, and are the sub-contractors qualified?

As discussed in the manufacturing quality control guide, process capability data (Cpk) should be available for critical manufacturing operations. A supplier that cannot produce Cpk data for a similar part is either not measuring capability or not measuring correctly.

Management and Workforce

The quality of a supplier’s operations is ultimately a function of their people and management culture. Observations to make:

  • Are quality and production in visible conflict (production pressure overriding quality holds)?
  • Is management knowledgeable about their own quality metrics? Can they cite current defect rates, on-time delivery, and open customer corrective actions?
  • What is workforce stability? High turnover is a quality risk — experienced operators know the process nuances that are not always captured in work instructions.

Red Flags During an Audit

Observations that should increase your risk rating for the supplier:

Resistance to access. A supplier that limits areas you can visit or is reluctant to show you specific processes is communicating something about what they do not want you to see.

Documentation that does not match practice. The most common gap: documented procedures that describe a process that operators do not actually follow. When operator practice differs from the procedure, the documented system is not operational.

No data on critical metrics. If a supplier cannot produce defect rates, customer complaint history, or corrective action response times, they are not measuring. What is not measured is not managed.

Calibration discrepancies. Out-of-date calibration on measuring instruments in use is an automatic red flag — any quality data produced with those instruments is suspect.

Material identification gaps. Unlabeled or improperly labeled work-in-progress, mixing of lots, or inability to trace a part back to its raw material source are traceability failures that matter in regulated industries and in quality investigations.

Excessive pending corrective actions. A large backlog of open corrective actions that are not being resolved signals either quality volume that exceeds their corrective action capacity, or a corrective action culture that opens reports but does not close them effectively.

Documenting Audit Findings

Audit findings should be documented in a format that supports the sourcing decision and creates a baseline for future audits.

Findings by category. Organize findings by audit area — facility, quality system, process capability, management — with specific observations noted. Generic findings (“quality system appears adequate”) do not support future audits.

Conformance vs. non-conformance. For each element audited, note whether the supplier is in conformance with your requirements or not, and if not, the specific nature of the nonconformance.

Risk rating. Apply a risk rating to each finding (major, minor, observation) based on impact to supply quality and continuity. Major nonconformances (e.g., no functional corrective action process) may be disqualifying. Minor nonconformances require documented corrective actions with timelines. Observations are improvement opportunities.

Supplier corrective action requests. For findings requiring correction, issue formal Supplier Corrective Action Requests (SCARs) with deadlines and verification requirements. Follow up to verify corrective actions are completed before beginning production.

Frequently Asked Questions

How long should a manufacturer audit take?

A comprehensive first-article audit of a new manufacturer typically takes 4–8 hours on-site, plus preparation and documentation time. Surveillance audits of familiar suppliers with known processes can be done in 2–4 hours. Complex, multi-process facilities may require multiple days.

Should we share the audit checklist with the supplier in advance?

Sharing the audit scope and general focus areas is reasonable and allows the supplier to prepare relevant documentation and arrange for the right personnel to be present. Sharing the detailed checklist in advance allows suppliers to stage the audit rather than revealing normal operating conditions — which defeats part of the purpose. Share the scope, not the detailed checklist.

Can third-party audit firms conduct supplier audits on our behalf?

Yes, and for international suppliers where travel is expensive, third-party audit firms provide cost-effective coverage. Quality assurance firms like Bureau Veritas, SGS, Intertek, and TÜV conduct supplier audits to customer-specified criteria. The limitation is that your team’s institutional knowledge of the supplier, built through the direct audit relationship, does not transfer to a third party. Use third-party audits for coverage, not as a permanent replacement for customer audits of critical suppliers.

What if a supplier fails the audit?

Document specific findings, issue corrective action requests with timelines, and set a follow-up audit schedule. Do not place production orders until critical findings are resolved. For major systemic failures, consider whether the supplier is a viable source at all — sometimes the finding is that the supplier is not the right fit for your requirements, and the appropriate outcome is to continue with alternative suppliers.

How do we handle a supplier that improves before the corrective action deadline?

Verify the improvement is genuine before closing the finding. Request documentary evidence of the corrective action (updated procedures, training records, calibration records), and for critical findings, conduct a follow-up visit or remote verification to confirm the change is implemented and sustained.

Further Reading from Authoritative Sources