Consent mode blocking analytics: diagnosing the problem

Symptoms

The clearest symptom is a sudden and steep drop in sessions, users, and events in GA4 that doesn't correspond to a change in actual traffic. You might see sessions drop by 60–90% overnight, or GA4 Realtime showing zero active users while your site is clearly receiving traffic. GA4's own data gaps report may also surface this as an unexplained gap.

A subtler symptom is that GA4 shows modelled conversions but zero or near-zero observed conversions from EU traffic segments — a sign that GA4 is running in "unconsented" mode and modelling rather than recording.

The root cause: blocking default state

When Consent Mode is implemented with a "denied" default state for analytics_storage, GA4 operates in an unconsented mode. In basic Consent Mode, this means GA4 sends cookieless pings with limited data. In advanced Consent Mode with modelling enabled, GA4 attempts to model unconsented behaviour.

However, if the consent update call (the one that fires after the user clicks Accept) is missing or malformed, the consent state never changes from "denied" — even for users who accepted. This means accepted users are still being tracked in unconsented mode, causing a significant data loss that is invisible in the GA4 interface.

How to diagnose the issue

Open Chrome DevTools on your site and open the Console. Accept the consent banner, then run: window.dataLayer.filter(e => e[0] === 'consent'). You should see both a "default" entry and an "update" entry, with the update showing "granted" for the appropriate parameters.

If you see only the "default" entry with no "update", or if the update entry shows "denied" for all parameters regardless of what the user chose, your CMP is not sending the consent update correctly.

Fixing without disabling consent

Do not "fix" this by removing Consent Mode or setting the default to "granted" for all users — this is a compliance risk for EU users. Instead:

1. Check your CMP's GTM integration and ensure the consent update callback is correctly mapped to a GTM trigger.
2. Verify that the CMP's GTM template version supports Consent Mode v2 (check for the two new parameters).
3. Test in GTM Preview mode to confirm the consent update trigger fires after banner interaction.
4. Re-run the GA4 Audits crawl to verify the fix is in place before releasing to production.