Why does the GA4 Promotions report show no revenue?

Two causes stack. First, GA4 does not carry promotion data between events; that is how the product behaves. Second, in most off-the-shelf e-commerce setups and retrofitted GTM containers the layer that closes that gap was never written. The `promotion_id` and `promotion_name` sent with `select_promotion` stay on that event only. For revenue to reach the report, the same values have to be written back onto the `purchase` event: preferably onto the matching products in the `items[]` array, at minimum at event level. GA4 falls back to the event-level value when it finds nothing at item level; if neither exists, the report stays limited to the impression and click columns.

Can I join banners to purchases in BigQuery without a carrier?

Partly. Pin the session with `user_pseudo_id` and `ga_session_id`, then tie the `add_to_cart`, `begin_checkout` and `purchase` events that follow the `select_promotion` timestamp to that promotion. This captures purchases completed in the same session. For purchases completed in another session or on another device, no key exists to join on.

How much promotion attribution is lost when consent is refused?

All of it. In advanced consent mode the refusing visitor's events land in BigQuery without an identity, so even the in-session sequence cannot be built. In basic consent mode no rows arrive at all. If the carrier is a cookie or `localStorage`, it cannot be written either, because it falls under the same permission category. The loss is therefore not partial; in that cohort it is complete.

Can I correct the loss by multiplying by the consent rate?

It can serve as a rough upper bound, but presenting the result as a measurement would be wrong. The purchasing behaviour of the refusing cohort does not have to match that of the accepting cohort, so scaling by the rate assumes a random sample, and that assumption has not been verified. The correct reading is to report promotion attribution as a lower bound and place the period's consent rate next to it.

When two different banners are clicked in the same session, which one gets the purchase?

Unless a rule is chosen, neither gets it cleanly. The query has to pick one explicitly: last click, first click, or an even split across every promotion touched. When the carrier is built, writing the last click into the main field and the full sequence of touches into a separate field keeps both readings open.

Does this measurement show that the banner caused the sale?

No. Both the in-session sequence and the carrier setup give correlation. The homepage hero is already shown to visitors with above-average purchase intent, so part of the relationship is selection effect. A causal claim needs a control group that never sees the banner, or a time-based holdout.

Linking a Banner Click to a Purchase: Promotion Events, the Carrier Layer and the Consent Boundary

In most e-commerce tracking setups and retrofitted GTM containers, GA4 promotion parameters do not travel between events, so view_promotion and select_promotion alone cannot be tied to a purchase. What BigQuery yields without a carrier, and what changes once one is in place, stays unclear. Add the consent flow and the uncertainty grows.

TL;DR

GA4 does not carry promotion parameters between events, and most e-commerce setups never close that gap. Without a carrier between the banner click and the purchase, the Promotions report shows impressions and clicks only, and the revenue column stays empty. The same join can be built in BigQuery, but only within a session; delayed and cross-device purchases do not surface that way. When consent is refused, the carrier and the join disappear together, because both depend on the same permission.

Get Access Sign In

Subscribers Only

The full article plus 22 more subscriber-only posts come with a subscription.

analytics

Subscribers Only

RELATED

Joining Google Ads and GA4 at Row Level: A Shared Key and a Two-Layer Reading

Connecting a Shopify Order to an Ad Click: Choosing a Join Key for Offline Conversions

GA4 Attribution Reports: Direct Inflation, Model Selection, and Finding the Real Source

Why Google Ads and GA4 Conversions Never Match: Counting Unit and Time Axis

GA4 Source/Medium Debugging: Detecting the Real Source with First-Party Cookies

Shopify Analytics vs GA4: Why Do They Show Different Numbers?

GTM vs sGTM vs Zaraz vs Gateway: I Tested All Four with Real E-commerce Data

Setting Up Umami with GTM in a Consent Compliant Way: The Transmission Threshold, the Decision Layer and the Replay Call

The Decisions You Lock In Before Importing Ad Cost Into GA4

GA4 Landing Page (not set): A Diagnostic Playbook