Supply Chain Sustainability Audit for Automotive Suppliers

We help from both sides of the supply-chain audit relationship: suppliers who have received an OEM request and need to verify their evidence — and OEMs or larger corporates who want to set up their own supplier audit programme for their entire supply chain.

What is a supply chain sustainability audit?

A supply chain sustainability audit is an independent verification that a supplier operates in line with sustainability requirements set by its customers — typically covering Scope 2 emissions (purchased electricity) and sometimes extending to Scope 3 categories. For automotive suppliers, the most common scope is verification that energy consumption is covered by renewable energy, using instruments such as Guarantees of Origin (GO), International Renewable Energy Certificates (I-REC), or Chinese Green Electricity Certificates (GEC).

The direct effect: supply chain decarbonization, reduction of the Product Carbon Footprint (PCF), and lower Scope 3 emissions on the customer side. A positive verification report is typically accepted by OEMs for multiple reporting years.

Standards we apply

ISAE 3000 (Limited Assurance) is the international standard for assurance engagements other than audits of historical financial information. We express a conclusion on your renewable energy claims. Required for mass balance allocations, dedicated meter setups, and whenever the verification needs to serve multiple customers.

ISRS 4400 (Agreed-Upon Procedures) is a factual-findings standard. We perform specific, pre-defined procedures on your green energy certificates and report factual findings without expressing an opinion. Efficient, typically 2 auditor days, and suitable for straightforward verifications.

Typical scope for automotive suppliers

• Green electricity contract verification — 100% renewable supply with Guarantees of Origin cancellation statements, matched to actual consumption.
• Mass balance allocation — when the OEM is one of several customers, we apply OEM Ratio = OEM Revenue / Total Revenue and verify that the corresponding share of renewable certificates is retired with a specific production reference.
• Dedicated meter setups — separate meter feeds the OEM production line. Requires on-site inspection.
• Multi-site, multi-country groups — consolidated engagement under ISAE 3000 across sites.

Same situation at any OEM supplier

Whether you deliver to ZF or to any other major automotive OEM: the underlying supply-chain verification logic is highly portable. Every OEM with a supply-chain decarbonisation programme asks its suppliers to demonstrate renewable electricity for the share of production delivered to that customer — typically through CDP Supply Chain, EcoVadis, or a proprietary supplier portal, backed by a third-party verified certificate chain.

Differences are in format (CDP, EcoVadis, proprietary portals) and in threshold definitions (revenue share, production share, tier of supply). We assess your specific customer requirement in the initial call and align the engagement scope so one audit serves multiple customer reports.

Typical evidence questions from OEM customers

• Are certificates retired with a production reference? GOs, I-RECs or GECs must carry a clear attribution to the customer-specific production line or legal entity.
• Does the retired quantity match the allocated share? For mass balance setups: ratio of customer revenue to total revenue, times total electricity consumption.
• Is the evidence third-party verified? Most OEMs require an independent assurance engagement under ISAE 3000 or ISRS 4400.
• Does the report match our reporting cadence? Typically valid multi-year, renewable upon material change in setup or supplier relationship.

Tier-1 and tier-2 suppliers, cross-border

Supplier position in the chain and site location matter for scope. Tier-1 suppliers often need a site-specific report tied to one production unit; tier-2 and deeper suppliers typically work with mass-balance allocations. We run the same audit logic across EU (GO/HKN), China (GEC, I-REC), India (I-REC) and Türkiye. One audit team, one engagement letter, one report — accepted by multiple downstream customers.

Setting up your own supplier audit programme

For OEMs and larger corporates that want to verify the supply-chain evidence of their own suppliers: we bring the proven ZF blueprint — the combination of Recommendation (full green-electricity coverage at group / entity / plant level) and Minimum Requirement (mass balance or dedicated meter) — and adapt it to your reporting cycle and evidence criteria.

What we deliver for programme owners:

• Audit methodology design — scope definitions, evidence requirements, threshold logic, acceptance criteria.
• Supplier guidance documents — what each supplier must prepare, which certificates qualify, how mass balance is calculated.
• Execution of the audit engagements — we run the individual ISAE 3000 / ISRS 4400 engagements with your suppliers, consistent with your programme rules. Reports delivered to you + the supplier.
• Ongoing programme governance — annual review, threshold updates, onboarding of new suppliers, coordination with CSRD / Scope-3 reporting.

You get a running audit programme without building internal audit capacity — structured the same way ZF structured theirs, or tailored to your specific supply-chain and regulatory requirements.

Why an independent WPG auditor?

As a licensed German Wirtschaftsprüfungsgesellschaft, we are subject to strict independence rules (§ 49 WPO), professional secrecy (§ 43 WPO, § 203 StGB), and the quality management standards of the Wirtschaftsprüferkammer and Institut der Wirtschaftsprüfer. The engagement letter includes an explicit independence statement. We do not provide bookkeeping, tax advice, or valuation services to our audit clients.

How to start

Book a free 30-minute consultation. We assess your setup (single site, multi-site, mass balance, dedicated meter), explain the ISRS 4400 vs ISAE 3000 choice, and provide a fixed-price quote before any engagement begins.