Sessions dropped 30% the week the new cookie banner shipped. Revenue did not move. Orders did not move. The warehouse is as busy as it was.
Nothing happened to your traffic. Something happened to your measurement of your traffic, and it is working as designed.
What Consent Mode actually does
Consent Mode is a signalling layer between your cookie banner and Google’s tags. Your consent management platform tells the tags what the visitor agreed to, and the tags adjust their behaviour.
Version 2, mandatory since 6 March 2024 for EEA and UK users, defines four signals:
| Signal | Controls | Added in |
|---|---|---|
analytics_storage | analytics cookies, so GA4 measurement itself | v1 |
ad_storage | advertising cookies | v1 |
ad_user_data | whether user data is sent to Google for advertising | v2 |
ad_personalization | personalised ads and remarketing | v2 |
The one that moves your GA4 numbers is analytics_storage. When it is denied, the GA4 hit does not happen. The visit is real; the measurement of it is not.
Basic vs Advanced — the choice that decides how much you keep
This is the decision most teams make by accident, and it has the largest effect on your data.
Basic mode. Google tags do not load until the visitor grants consent. Decline, and nothing is ever sent. Clean, strict, and simple to defend. It enables conversion modelling in Google Ads but blocks behavioural modelling in GA4, so your GA4 reports keep the full hole.
Advanced mode. Tags load immediately with consent defaulted to denied, and send cookieless pings — no identifiers, no cookies, just an anonymous signal that something happened. Google uses those pings to model the behaviour it cannot observe directly. Advanced is eligible for behavioural modelling in GA4 as well as conversion modelling in Ads, subject to Google’s data thresholds, and reporting suggests it recovers a substantial share of otherwise-lost conversions.
The trade is not really technical. Advanced sends an anonymous signal before consent is granted; Basic sends nothing at all. Which of those your organisation is comfortable with is a legal question, and it should be answered by whoever owns privacy compliance rather than by whoever owns the tag container.
Modelled data is an estimate, and should be labelled as one
If modelling is active, some of what you are reading was inferred rather than observed. Google estimates the behaviour of users who declined based on patterns from users who accepted.
Three consequences worth internalising:
- Modelling has a minimum volume. It only activates once a property meets Google’s thresholds. Smaller sites get the data loss without the recovery.
- It is directional, not exact. Useful for “is this trending up”. Unreliable for reconciling against a CRM or order table, and that mismatch is not a bug to chase.
- It appears without a label in most reports. You cannot tell modelled from measured by looking.
Diagnosing a consent-shaped drop
The signature is specific: traffic falls, business outcomes do not.
Check in this order:
- Correlate the drop with a deployment date. A consent banner launch or a CMP configuration change on the same day is close to conclusive.
- Compare against a source outside GA4 — server logs, order counts, Search Console clicks. Search Console is especially useful here because it is measured on Google’s servers and consent cannot block it. A widening gap between Search Console clicks and GA4 organic sessions is a strong consent signal.
- Segment by geography. If the drop is concentrated in the EEA and UK and your other markets are flat, that is the answer.
- Check which mode you are actually in. Many teams believe they run Advanced and are running Basic, because the CMP template defaulted that way.
That fourth one is worth checking today. It is the single most common finding, and switching modes is a configuration change rather than a rebuild.
What is genuinely unrecoverable
Consent-blocked visits are not stored anywhere. There is no export, no backfill, no BigQuery workaround, and no third-party tool that can retrieve them — the data was never collected.
That includes ours. GA4 Simple View reads the GA4 Data API, so it inherits precisely the same gap and the same modelling as the GA4 interface. We cannot show you visitors Google never recorded, and neither can anything else.
The one thing we do make easier is step 2 above: Search Console clicks and GA4 sessions sit on the same Search tab over the same date range, so tracking the ratio between a consent-immune source and a consent-affected one is a glance rather than a spreadsheet. When that ratio moves sharply, you have your answer — and the fix lives in your consent platform, not in any analytics tool.
Questions people also ask
Each answer stands on its own, so it still makes sense quoted somewhere else.
What are the four Consent Mode v2 signals?
ad_storage and analytics_storage came with the original Consent Mode. Version 2 added ad_user_data, which controls whether user data is sent to Google for advertising purposes, and ad_personalization, which governs remarketing. All four must be implemented for EEA and UK users to keep using personalised advertising and measurement features.
What is the difference between Basic and Advanced Consent Mode?
In Basic mode the Google tags do not load at all until the user grants consent, so nothing is sent for those who decline. In Advanced mode the tags load immediately with consent defaulted to denied and send cookieless pings, which Google uses to model the behaviour it cannot directly observe.
Which mode should I use, Basic or Advanced?
Advanced recovers substantially more data because it enables behavioural modelling in GA4 as well as conversion modelling in Google Ads, with reports suggesting recovery of a large share of otherwise lost conversions. Basic is simpler and stricter, sends nothing before consent, and loses more. The choice is a legal and risk decision as much as a technical one.
Why did my GA4 sessions drop after adding a consent banner?
Because visitors who decline analytics cookies no longer produce GA4 hits, while your actual traffic is unchanged. A drop that coincides with a banner launch and is not matched by a drop in revenue, orders or server logs is almost certainly consent-related rather than a real traffic loss.
Is modelled data in GA4 real data?
It is an estimate, not an observation. Google models the behaviour of users who declined consent based on patterns from users who accepted, and modelling only activates once a property meets Google's data thresholds. It is directionally useful for trends and unreliable for reconciling exact counts against another system.
When did Consent Mode v2 become mandatory?
It has been required since 6 March 2024 for anyone using Google Ads, GA4 or Google Marketing Platform features with EEA or UK users. Without the four signals implemented correctly, you lose the ability to build audiences from those users and to share them with Google Ads for remarketing.
