GTM Custom HTML Tag Firing But Not Tracking: Debug Checklist

GhazanfarApril 17, 20262 min read

Your GTM Custom HTML tag shows a green checkmark in Preview mode — the tag fires. But no data appears in your analytics tool. This is one of the most frustrating GTM debugging scenarios. The tag runs, but something silently fails inside it. This checklist walks through every possible failure point in order of likelihood.

Verify the Tag Actually Executed Its Code

GTM Preview showing “Tag Fired” only means the JavaScript was injected — not that the code inside ran successfully. Add a console.log as the first line:

<script>
console.log('[GTM Debug] Custom HTML tag executing');
gtag('event', 'test_event', {event_category: 'debug'});
console.log('[GTM Debug] gtag call made');
</script>

Check for JavaScript Errors

ReferenceError: gtag is not defined — GA4 config tag hasn’t loaded yet; use tag sequencing
TypeError: Cannot read property of undefined — a dataLayer variable doesn’t exist on this page
SyntaxError — malformed JS inside script tag; validate with an online linter
Content Security Policy blocked — check Console for CSP violations

Tag Firing Order Problems

Use Tag Sequencing: Advanced Settings → Tag Sequencing → fire config tag before custom HTML tag. Or wrap code defensively:

function waitForGtag(cb, n) {
  n = n || 10;
  if (typeof gtag === 'function') { cb(); }
  else if (n > 0) { setTimeout(function(){ waitForGtag(cb, n-1); }, 200); }
}
waitForGtag(function() {
  gtag('event', 'your_event', {event_category: 'test'});
});

GTM Custom HTML Tag Firing But Not Tracking: Debug Checklist

Network Request Check

Open DevTools → Network → filter “g/collect”. Fire the event and watch for requests to google-analytics.com/g/collect. If you see the request, GA4 received the hit. If not, the gtag call itself failed.

Variable Not Populating

var userId = '{{dlv - user_id}}';
if (!userId || userId === 'undefined' || userId === '') {
  console.warn('[GTM] user_id not available, skipping');
  return;
}
// proceed with tracking

Ad Blockers and Extensions

Test in incognito with no extensions. uBlock Origin, Privacy Badger, and similar extensions block GTM entirely or block analytics calls. If it works in incognito but not regular mode, an extension is blocking it — this affects real users too.

Debug Checklist

☐ Add console.log as first line — confirm execution
☐ Check Console for JS errors
☐ Verify tag firing order — config before custom HTML
☐ Check Network for outgoing collect requests
☐ Verify all GTM variables have values at fire time
☐ Test in incognito with no extensions