Read the dataLayer in preview

August 12, 2026 view_item_listview_item

What we are after

A dataLayer is diagnosed in the data model of GTM preview, not in the browser's Network tab. The network request shows what left; the data model shows what GTM read — and that is where the defects live.

The walkthrough

1. In your container, click Preview and enter https://static.ttdlab.com/en/shop.
2. Once the preview window connects, look at the left column: a view_item_list event is already there.
3. Select it, then open the Data Layer tab — not Variables.

You should see an ecommerce object with item_list_id: "shop_all" and eight entries in items, each carrying its index.

What to look at next

Click through to a product page. Preview shows a select_item, then a view_item. Select the view_item and unfold its ecommerce: it must contain exactly one item.

If it contains eight, you have just reproduced the defect that the next tutorial covers in detail — and that is why this site pushes { ecommerce: null } before every event.

Why the Network tab is not enough

A GA4 tag firing with wrong items produces a perfectly valid request. Status 204, well-formed parameters, nothing to report. The only place the error is visible is the data model, before the send.