One of the unfortunate misunderstandings regarding the wonderful Google Tag Manager Preview mode is what it actually means when GTM reports “Tags Fired On This Event”. For many, this seems to indicate that whatever code the Tag was meant to execute also completed successfully. However, this is not the case.
Tip 19: Debugging Tag execution vs. actual requests
Let’s get the distinction straight right away:
This is a very clear distinction, and it basically means that looking at the debug panel alone, you cannot debug whether or not the execution of your Tags is performing expectedly.
For example, if you have some Google Analytics blocker running in the browser, GTM’s debug panel will tell you that the Tag has “fired”, but the browser will prevent the Tag code from completing its request to Google Analytics servers.
How to debug the whole process
Network requests can be debugged with your browser’s Developer Tools:
Chrome / Mac: Open the Developer Tools with Cmd + Opt + I and navigate to “Network”
Chrome / Windows: Open the Developer Tools with F12 and navigate to “Network”
Firefox / Mac: Cmd + Opt + Q
Firefox / Windows: Ctrl + Shift + Q
What you’ll see is the list of requests your browser has made during the lifespan of the page. You can locate a specific type of request using the filter feature (Chrome). For example, to see hits to the Universal Analytics endpoint, I would filter for requests with the word “collect”. That’s because collect is the URI where all Universal Analytics hits are dispatched to.
Next, I’ll focus on the Status column. Typical errors are e.g. 404 (not found) and 500 (internal server error). A successful request would have the response code 200.
If you want an even clearer view to what’s happening on the site, complete with an analysis of
dataLayer state, take a look at my favorite debugging tool, WASP.
Debug Panel tells you when a Trigger causes a Tag to be injected on your site.