In some cases, Google Ads Conversion tags appear with a status 'failed' in Google Tag Manager preview mode, even though the tag fired correctly and the conversion was received by Google Ads.
This is a false positive: the error in GTM preview mode does not impact the actual tracking or reporting in Google Ads.
Background
The issue typically occurs when the Conversion Linker tag writes the click ID to a cookie with a non-standard name. Google Ads expects to find this ID in a cookie prefixed with _gcl. If that name is different or missing, the GTM preview mode may label the tag as "failed", even though the backend processing still works.
How to fix it
To resolve the false "failed" status, manually override the cookie prefix in both tags:
Conversion Linker tag
- Go to your Conversion Linker tag
- Under Advanced Settings, enable Override cookie settings
- Set the cookie prefix to
_gcl1(or another consistent value)
Google Ads Conversion or Remarketing tag
- Open the corresponding Google Ads tag
- Scroll to Conversion linker settings
- Enter the same cookie prefix (
_gcl1)