Which pages should I monitor for tags?

Pick at least one URL per template: the homepage, a product or service page, a key landing page and a blog post. Thank-you or order confirmation pages often matter most for conversions, but many can only be reached after a form submission or purchase, so an external check may not be able to load them.

Is it a problem to install GA4 both directly and through GTM?

Yes. If the gtag.js snippet and a GA4 tag in GTM both send page views to the same measurement ID, sessions and page views are counted twice. When you move GA4 into GTM, remove the hardcoded snippet or the plugin that added it.

Does Meta's Conversions API replace the Meta Pixel?

Usually not. The Conversions API sends events from the server and is typically run alongside the browser pixel, with an event_id used to deduplicate the two. If the pixel disappears, browser-side events and audience signals from the site are still lost.

Can a tag be on the page and still send no data?

Yes. The script can be blocked by a consent banner until the visitor accepts, run in Consent Mode with storage denied, or be present as a GTM container whose GA4 tag is paused. That's why an alert about a changed cookie banner is worth reading next to any tag alert.

How much does marketing tag monitoring cost with Deltio?

Tag monitoring is included in every plan, alongside the SEO, sitemap, uptime, SSL and domain checks. Starter covers up to 50 sites and 5,000 URLs for £24 / €29 / $31 a month, or £20 / €24 / $26 a month billed annually. Professional covers up to 200 sites for £49 / €59 / $64 a month (£41 / €49 / $53 billed annually), and Enterprise up to 500 sites for £119 / €139 / $149 a month (£99 / €116 / $124 billed annually). Every plan has a 14-day free trial.

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How to Know the Moment a Marketing Tag Disappears (GA4, GTM, Meta Pixel)

A marketing tag monitoring tool checks your pages on a schedule and tells you when a tag such as GA4, Google Tag Manager or the Meta Pixel is no longer there, or no longer matches what was there before. Tracking rarely breaks with an error message. A deploy, a theme update or a new cookie banner removes the snippet, and the first sign is a flat line in GA4 or a campaign reporting zero conversions days later.

Missing data can't be backfilled. If GA4 misses a week of sessions, that week stays empty in every report, and any ad campaign optimising towards conversions spent the week optimising towards nothing.

What each tag looks like in the page source

Every common tag leaves recognisable traces in the HTML. A manual check and an automated one both rely on them.

Tag	What to look for in the source	ID format
GA4 (gtag.js)	A script loaded from `googletagmanager.com/gtag/js?id=` and a `gtag('config', ...)` call	`G-` followed by letters and digits
Google Tag Manager	A script loading `googletagmanager.com/gtm.js?id=`, plus a `<noscript>` iframe just after `<body>`	`GTM-` followed by letters and digits
Meta Pixel	`connect.facebook.net/en_US/fbevents.js` and an `fbq('init', ...)` call	A long numeric pixel ID

When GA4 is deployed through GTM, there's no G- ID in the source, only the container. That's normal, and it's one way a page can look fully tagged while GA4 receives nothing, because the GA4 configuration lives inside the container rather than on the page. For a step-by-step manual check, see how to check GA4 is installed.

How tags go missing

On WordPress, GA4 is often added by a plugin such as Site Kit or pasted into the theme's header.php. Deactivating the plugin while debugging something else, or switching theme, takes it away. On Shopify, code pasted into theme.liquid belongs to that theme, so publishing a new theme drops it unless someone copies it across, while pixels set up through the Facebook & Instagram app or the Customer events section survive a theme change. Custom-built sites lose tags in rebuilds, rollbacks and merges that undo an earlier hotfix, and migrations sometimes ship the staging container ID to production.

A changed ID is the hardest case to spot. The snippet is present and data flows, but into a test property or into another client's account because a template was copied from a different project. Nothing is missing, so nothing looks broken.

Consent is the other big cause. With a consent management platform (CMP) such as Cookiebot, OneTrust or iubenda, tags are blocked, or run in Google Consent Mode with analytics_storage and ad_storage set to denied, until the visitor accepts. Replace the CMP or change its configuration and tags that are still in the HTML may stop sending data. The opposite also happens, with tags firing before consent, which is a compliance problem rather than a data one.

Checking by hand, and where that stops working

For one site the tools are free. GTM's Preview mode (Tag Assistant) shows which tags fire on each page, GA4's DebugView shows events arriving, and the Meta Pixel Helper extension for Chrome lists pixel events. The DevTools Network tab, filtered for collect or facebook, works in any browser.

That's fine once. Across twenty client sites it falls apart. You check the homepage, and the tag went missing on the product template. You check on Monday, the tag broke on Tuesday, and someone fixed it on Friday after the campaign had spent its budget. It's the same pattern as every other silent SEO change, where a process that depends on someone remembering to look eventually misses something.

When an alert arrives

Work out the scope. Gone from one template points to that template. Gone from every URL points to the global header include, the plugin, or the GTM container snippet.
Compare the date with the last release, theme publish or plugin update. That often settles it.
If the container is still on the page, open GTM's version history. A tag paused during debugging and then published by mistake shows up there. After that, check the CMP settings.
Mark the affected dates in client reporting and treat campaign data from that window with suspicion.
Move the snippet somewhere a theme update can't reach, such as a child theme, a plugin or the platform's native integration.

Tag checks in Deltio

Deltio checks the pages it monitors for GA4, Google Tag Manager and the Meta Pixel and compares each daily check with the one before, so a tag that has disappeared or changed is reported by Slack or email for that site. It also watches the cookie consent banner, which means a CMP that vanishes or gets replaced shows up next to the tag change it probably caused. What it can't see is whether a tag configured inside a GTM container is published and firing. For that, Preview mode is still the right tool.

These checks run alongside the SEO checks described in SEO change alerts, which is how agencies monitor client websites for changes without opening thirty tabs every Monday. Every plan comes with a 14-day free trial.

Frequently asked questions

Which pages should I monitor for tags?: Pick at least one URL per template: the homepage, a product or service page, a key landing page and a blog post. Thank-you or order confirmation pages often matter most for conversions, but many can only be reached after a form submission or purchase, so an external check may not be able to load them.
Is it a problem to install GA4 both directly and through GTM?: Yes. If the gtag.js snippet and a GA4 tag in GTM both send page views to the same measurement ID, sessions and page views are counted twice. When you move GA4 into GTM, remove the hardcoded snippet or the plugin that added it.
Does Meta's Conversions API replace the Meta Pixel?: Usually not. The Conversions API sends events from the server and is typically run alongside the browser pixel, with an event_id used to deduplicate the two. If the pixel disappears, browser-side events and audience signals from the site are still lost.
Can a tag be on the page and still send no data?: Yes. The script can be blocked by a consent banner until the visitor accepts, run in Consent Mode with storage denied, or be present as a GTM container whose GA4 tag is paused. That's why an alert about a changed cookie banner is worth reading next to any tag alert.
How much does marketing tag monitoring cost with Deltio?: Tag monitoring is included in every plan, alongside the SEO, sitemap, uptime, SSL and domain checks. Starter covers up to 50 sites and 5,000 URLs for £24 / €29 / $31 a month, or £20 / €24 / $26 a month billed annually. Professional covers up to 200 sites for £49 / €59 / $64 a month (£41 / €49 / $53 billed annually), and Enterprise up to 500 sites for £119 / €139 / $149 a month (£99 / €116 / $124 billed annually). Every plan has a 14-day free trial.