Preview mode and the network tab

You've built a few tags. Before any of them go live, this lesson is the checklist you run — every time, on every change, for as long as you do this work.

Preview mode: GTM's side of the story

Click Preview in the container. Enter the URL, connect. Two windows: your site with a "Tag Assistant connected" badge, and the debug window.

The debug window's left column is the event timeline — every event GTM saw on this page, in order: Consent Initialization, Initialization, Container Loaded, DOM Ready, Window Loaded, then your clicks and dataLayer pushes. Click any event and the right side shows:

• Tags — which tags fired on this event and which didn't. Click a tag to see the resolved values of every parameter. This is where undefined in a value field jumps out.
• Variables — every variable's value at that moment. If a Data Layer Variable is empty here, the push didn't have the key (or the key path is wrong: ecommerce.value not ecommerce_value).
• Data Layer — the array state after this event. The source of truth for "what did the site actually send."
• Errors — JavaScript errors from Custom HTML tags. Read this tab; they're silent everywhere else.

What Preview cannot tell you: whether the request left the browser, and whether it was accepted. It shows intent.

The network tab: what actually left

DevTools → Network → filter collect (GA4) — or gtm.js, tr/? (Meta), bat.bing (Microsoft), whatever vendor you're checking. Keep it open while you click around.

For each request:

• Status — 204 or 200 is accepted. 0/(blocked) means an extension or a content-security policy stopped it. A 4xx means the vendor rejected it.
• Payload / Query — the parameters. en, tid, ep.*, epn.* for GA4. Confirm the value is a number with no currency symbol, the event name is exact, the tid is the right property.
• Count — one action, one request. This is the check that catches duplicate installs, and it's the one people skip.

Preview says the tag fired but there's no request? Look for a JS error earlier on the page (Errors tab, then the Console). Look at consent state — a denied analytics_storage suppresses GA4 hits entirely in basic Consent Mode. Look for an ad blocker in this browser.

DebugView: the receiving end

GA4 → Admin → DebugView. While Preview is connected, your browser is in debug mode and your events stream here within seconds, with every parameter expandable. If a request shows 200 in the network tab but nothing appears in DebugView: wrong tid, or an internal-traffic / developer-traffic filter is excluding you (check Admin → Data filters).

The three-view habit

Before you publish anything:

1. Preview — the tag fired on the right event, and every parameter resolved to a real value.
2. Network — one request per action, right endpoint, right tid, 2xx status.
3. DebugView — the event arrived with the parameters you expect.

Then Submit → Publish, with a version name that says what changed (v14 – add cta_click event). Versions are your undo button; a good name is what makes the undo findable when something breaks two weeks later.

Do it on the sandbox

Point Preview at the sandbox, add a product to the cart, and walk the three views for add_to_cart. Then go to checkout, place the order, and do it again for purchase on the thank-you page — including the refresh test: reload the thank-you page and confirm no second purchase request appears. The sandbox guards against it; your real site needs to as well.

Now reload the sandbox home page with the scanner: tagintact.com/tools/tracking-health-scanner. It's the network-tab check, run from outside, on any page, in ten seconds. Module 2's quiz is next.