Guide 04 · Debugging · GA4 + Tag Manager
Tag Assistant Preview: read Tags Fired and Tags Not Fired like a list of suspects
Preview lists every tag under Tags Fired or Tags Not Fired for each event. A tag under Not Fired names its own reason — read the trigger condition, not the tag.
After the steps below, the event you care about shows the GA4 tag under Tags Fired, and every tag under Tags Not Fired has a named condition that did not match.
Download the check: the debugging check (JSON) (MD5 b65f4d972acd0dd6085039902ae2ac55, 525 bytes). It lists the checkpoints from this guide and the worked check, with the figures recomputed. The file was built for this guide and validated structurally — it was not exported from a live container or a live property.
What you need
- A Tag Manager workspace with the tag in it. Preview tests the workspace draft, so the tag and its trigger must exist in the workspace you open Preview from.
- Your site URL. Preview opens your site in a new window with the draft container loaded, and only in the browser where you started it.
- The event you want to test. A page load, a click, a form submit — know which event the tag should fire on before you start, so you can pick it out of the left-hand event list.
- The trigger, written down. The trigger type and its conditions, copied from the tag. You will compare them against what Preview says did not match.
The trace
The worked check: the figures below were recomputed before publication, and the note under the table shows the working.
| Check | Figure |
|---|---|
| Events clicked through in the test session | 6 |
| Events where the GA4 tag fired | 5 |
| Events where the tag sat under Tags Not Fired | 1 |
| Share of events where the tag fired | 83.33% |
Six events were clicked through in the test session and the tag fired on 5 of them, so 5 divided by 6 is 83.33%. The one event where it did not fire was a form submit whose trigger waited for a Click event, so the fix was the trigger, not the tag. A tag that fires on 5 of 6 events is not broken; it has one wrong condition.
- Start Preview from the workspace. In Tag Manager, open the workspace that holds the tag and click Preview at the top right. Tag Assistant opens in a new tab. Enter your site URL and click Connect. Your site opens in a new window marked Connected, and the Tag Assistant tab shows the debug view.
- Click through the site the way a visitor would. Load pages, click the button, submit the form — whatever the tag is supposed to follow. The debug view adds one entry per event to the left-hand list as you go. Do the exact action the tag waits for, not a near neighbour of it.
- Pick the event and read the two lists. Click the event in the left-hand list. The debug view shows Tags Fired and Tags Not Fired for that event. Your tag must be under Tags Fired. If it is under Tags Not Fired, click it: Preview shows which trigger condition did not match, in plain words.
- Compare the failed condition with the trigger. Open the trigger in Tag Manager and read its conditions side by side with what Preview reported. A trigger waiting for a Click event never fires on a form submit; a trigger waiting for a page path of /checkout never fires on /checkout/thank-you. Fix the condition that Preview named, not the whole tag.
- Run the event again and watch the lists change. With the trigger fixed, repeat the same action in the same Preview session. The tag should move from Tags Not Fired to Tags Fired on that event. If it does not move, the condition you changed was not the one Preview named — read it again.
Dead ends
DEAD END — READING THE WRONG EVENT. The debug view lists one Tags Fired list per event. Reading the list for the page load when the tag waits for a form submit proves nothing. Click the exact event the tag should fire on before you judge it.
DEAD END — PREVIEW IN ONE BROWSER, TESTING IN ANOTHER. Preview only appears in the browser where you started it. If you click through the site in a different browser, the debug view stays empty and the tags look dead. One browser, start to finish.
DEAD END — FIXING THE TAG WHEN PREVIEW NAMED THE TRIGGER. A tag under Tags Not Fired is not a broken tag. Preview names the condition that did not match, and that condition lives in the trigger. Rebuilding the tag leaves the same trigger in place and the same result.
End state
The event list read one event at a time, each tag placed under Fired or Not Fired, and each Not Fired tag traced to the exact trigger condition that did not match. A tag that fires on the right event in Preview will fire on the live site once the version is published — Preview tests the draft, publishing is what ships it.
Sources
- Tag Manager Help — Preview and debug containers — Preview and debug mode, the debug interface showing which tags fired, and what triggered or did not trigger a firing
- Analytics Help — Monitor events in DebugView — DebugView as the next check once a tag fires, to confirm GA4 received the event
Checked 11 November 2026. The click paths above are the ones in Google's help pages, opened 11 November 2026. What was run: the fired share, recomputed — 5 fired of 6 tested events is 83.33%, with 1 event not fired because the trigger waited for a Click event. No step in this guide was run inside a live Analytics property or a live Tag Manager container, because both need a Google sign-in and this site does not sign in as anyone. Google moves menu items without warning; if a path on this page no longer matches, the linked help page is the current map.