Google Ads Lost 40% of Its Conversion Data in a Single Week. Here's What Happened.

The situation

A German home goods brand was running €28K/month on Google Ads. In March 2024, Google enforced Consent Mode v2 for all EU advertisers. Within 10 days, tracked conversions dropped 40%. Smart Bidding began degrading visibly. Their in-house team had a consent banner — but had never heard of Consent Mode.

The problem

Their Cookiebot banner correctly blocked analytics cookies for users who declined consent — but it was not sending the corresponding Consent Mode signals to Google's gtag.js. Without these signals, Google could not apply behavioral modeling to estimate conversions from non-consenting users. In Germany, where 61% of users decline cookie consent, this meant 61% of all conversion data was simply disappearing — and Smart Bidding was learning from the atypical minority who accepted cookies.

What we did

1. Audited the Cookiebot implementation — confirmed zero Consent Mode parameters were firing on page load by checking the browser console
2. Implemented full Consent Mode v2 with Cookiebot: configured granted/denied signals for analytics_storage, ad_storage, ad_user_data, and ad_personalization on every page load based on user consent status
3. Enabled Google behavioral modeling in GA4 and Google Ads — allows Google to estimate conversions from non-consenting users using patterns from consenting cohorts
4. Added server-side Enhanced Conversions as a parallel layer for the 39% who do consent — hashed PII improves match quality
5. Built a GA4 consent dashboard to monitor weekly consent rates and modeled conversion percentages

Why it worked

Consent Mode v2 does not track users who decline — it gives Google the signals to model them using privacy-preserving aggregation. Without it, declining users are absent from the dataset and Smart Bidding learns from a biased sample. This is now mandatory for all EU advertisers. The majority are still implementing it incorrectly.