What happens if Consent Mode has no default state?
Google tags fire with their built-in defaults, which means granted. Until your banner sets a consent state, tags behave as if the visitor said yes to everything: analytics cookies get set, ad storage is enabled, and personalization signals go out. That is the opposite of what consent law requires in the EU and UK. The default state exists so tags start denied and only upgrade when the visitor chooses. Skip it and your banner is decoration.
Why the timing matters
The default command has to run before any Google tag loads. Tags read the consent state at load time and act on it immediately, so a default set even a second late leaves a window where data collection runs unrestricted. This is why consent platforms push the default into the head of the page, ahead of the tag manager snippet. Moving it later to improve page speed scores quietly breaks the whole model.
What breaks without it
Three things. First, compliance: regulators and consent-mode verification tools flag the missing default because pre-consent storage is exactly what the law forbids. Second, data quality: your modeled conversions lean on the behavioral modeling that only kicks in when denied states are properly signaled. Without the default, Google sees granted traffic and ungranted traffic mixed together and the modeling degrades. Third, debugging becomes miserable, because the tag assistant shows consent granted before any user interaction and nobody can tell whether the banner works.
Getting it right
Set the default to denied for ad_storage, analytics_storage, ad_user_data, and ad_personalization in every region where consent is required, and do it in the page head before the Google tag. Then let the banner update the state on choice, and persist that choice so returning visitors skip the denied start. Test with a fresh profile and watch the network requests: before any click, you should see cookieless pings only. That is the whole test. If you see full cookies before consent, the default is missing or misplaced.