Fourteen months of GA4 showing zero revenue after the UA migration

The situation

A French luxury wine brand migrated from Universal Analytics to GA4 in June 2023 (UA sunset deadline). Their developer installed the GA4 tag but didn't migrate any of the UA ecommerce implementation. GA4 showed traffic and page views but revenue was $0 and no purchase events appeared.

The problem

Universal Analytics used the old dataLayer ecommerce schema (ec:addProduct, ec:setAction 'purchase', etc.). GA4 uses a completely different schema (items array, ecommerce.purchase object). The existing GTM setup was pushing UA-format ecommerce data that GA4 simply ignored. The result was 14 months of zero revenue data in GA4 while the team made decisions on traffic metrics alone.

What we did

1. Audited GTM container — found 8 UA ecommerce tags with no GA4 equivalents
2. Built GA4 ecommerce data layer with correct schema: view_item, add_to_cart, begin_checkout, purchase — all using items[] array format
3. Configured GA4 purchase event with revenue, currency, transaction_id, and item-level detail (item_id, item_name, price, quantity)
4. Validated via GA4 DebugView — all events appearing with correct parameters within 24 hours of implementation

Why it worked

UA to GA4 migration is not just installing a new tag — it's rebuilding the entire ecommerce data layer to match GA4's schema. Many businesses installed GA4 alongside UA and assumed they were migrated. They weren't. The tag fires, GA4 gets traffic data, but without the ecommerce-specific schema, all purchase events are silently dropped.