Say that in February Google Ads claimed 270 of the 300 sales your store logged, and that in April, with the store unchanged, it claims 180. For the same spend, every sale now looks half as expensive again to Google. That is the cost it bids with.
Nobody touched the campaigns. What changed in March was the cookie banner, and with it what Google is allowed to count. That permission travels in something Google calls Consent Mode, and when it is wired wrong, no dashboard shows an error. Here is how to check it yourself in the browser, before someone cuts budget over a drop the campaigns did not cause.
What the banner tells Google
Every time someone lands on your site, the banner hands Google's tags four permissions. In plain words: whether cookies can be stored for advertising, whether cookies can be stored for analytics, whether Google can use that person's data for ads, and whether it can show them personalised ads. The last two are what version 2 added, and since March 2024 Google has required them to measure visitors in the European Economic Area and show them ads again.
There are two ways to set it up. In what Google calls basic mode, the tags wait for the visitor to accept, and if they decline, nothing goes to Google. In advanced mode, the tags check in from the first second with everything denied and no cookies stored, and Google uses those check-ins to estimate the conversions it cannot see.
Which one you use is a call for whoever owns privacy on the site. Your job is to know which one you actually have, because a cookie tool that blocks the whole tag quietly turns an advanced setup into a basic one.
Five minutes in the Network tab
Open your site in a Chrome incognito window and leave the banner alone. Press F12, go to the Network tab and type collect into the filter. Each row is a call to Google. Click one and look for the gcs parameter in the address. It always starts with G1; the two digits after it are the advertising and analytics permissions, 0 for denied and 1 for granted.
We ran this on our own blog on 15 October 2026. Before anyone touched the banner, the page view was already going to Google Analytics with gcs=G100, both cookies denied: that is what advanced mode looks like. After clicking accept all, the next calls switched to G111 and the _ga Analytics cookie appeared. After clicking reject all, no new call went out and the browser held no Google cookies.
On your own site, two results are worth raising the same day. If nothing goes out until you accept, you are on basic mode, and if your privacy owner thought you were on advanced, Google has not been able to estimate anyone who declines. And if the calls still say G100 after you accept, the banner is not passing the answer on: those people said yes and still do not count.
The two new signals, in Tag Assistant
gcs only shows the two cookie permissions. The two from version 2 are easier to read in Tag Assistant (tagassistant.google.com). Connect your site, accept all in the banner and open the Consent tab on the next event: all four permissions should read granted.
If two read granted and two still read denied, the banner was built for version 1 and never updated. Conversions keep arriving, which is why nobody notices, but Google cannot show ads again to those European visitors, even though they accepted everything.
In the same tab, check the first event on the page, the initialisation one. The default state has to show up there, before any tag fires. If it shows up later, the first tags go out without knowing what they are allowed to do. In Google Tag Manager the fix is to move the banner tag to the trigger that runs before all the others, called Consent Initialization. Any site migration or change of cookie tool can reshuffle that loading order, so this check gets repeated after each one.
Put a date on the gap with your own orders
The browser tells you whether the setup is right today. To find out when it broke, divide Google Ads conversions by the orders or leads your own system logs, month by month. In the opening example, February comes out at 90% and April at 60%. If that percentage drops in the month the banner or the site changed, the gap is in measurement. How to line up the sources is in why Google Ads and GA4 conversions do not match.
Do not count on modelling to fill it in. Google sets out the bar in its GA4 help: to estimate the people who decline, your site needs around 1,000 events a day from people who decline, for a week. On top of that it needs around 1,000 visitors a day who accept, on at least 7 of the last 28 days. A site below that volume gets nothing back, and one above it models from whatever the banner lets through.
That is why the signal comes first. Until the banner passes all four permissions correctly, any debate about attribution models or windows is being had over conversions that are missing.
Frequently asked questions
What is Google Consent Mode v2?
It is how a website passes Google's tags what each visitor accepted in the cookie banner. It uses four signals: ad_storage and analytics_storage (advertising and analytics cookies), and ad_user_data and ad_personalization (use of data for ads and personalised ads). Version 2 added the last two, and Google has required them since March 2024 to measure and remarket to visitors in the European Economic Area.
What is the difference between basic and advanced Consent Mode?
In basic mode the tags do not load until the visitor accepts, and nothing is sent if they decline. In advanced mode they load with everything denied and send cookieless check-ins, which Google uses to estimate the conversions it cannot see. You can tell them apart in the browser's Network tab: in advanced mode a collect call with gcs=G100 goes out before the banner is touched; in basic mode nothing does.
How do I check that Consent Mode v2 is set up correctly?
Two checks. In the Network tab, the gcs parameter should go from G100 to G111 when you accept all. In Tag Assistant, all four permissions should read granted after accepting, and the default state should appear before any tag fires. If the share of orders that Google Ads records dropped in the month the banner changed, that is where the problem is.
Does Google recover lost conversions through modelling?
Only with volume. GA4 modelling requires at least 1,000 daily events with analytics_storage denied for 7 days and at least 1,000 daily users with analytics granted on 7 of the previous 28 days, and even then it is not guaranteed. Below that, what is not measured is not recovered.
Open the Network tab before your next budget meeting
Five minutes with the banner and the collect filter tell you whether Google can see the people who accept. If it still says G100 after you accept, the drop is not coming from the campaigns. Every weekday morning, one check like this in your inbox, in a one-minute read.