Cross-Domain Tracking in GA4: Diagnosis and Solution

Written By Ali Izadi

The Case File

When users navigate between your main website and a separate domain—say, from yourcompany.com to shop.yourcompany.co.uk or from your marketing site to a third-party payment processor—GA4 treats each domain transition as a new user starting a new session. Without proper cross-domain tracking configuration, the user journey fragments. What should appear as one continuous path becomes multiple disconnected sessions, each with a different attribution source.

This check measures whether your GA4 property is configured to maintain user and session continuity across multiple root domains. While subdomains are automatically tracked (e.g., blog.yourcompany.com and shop.yourcompany.com work out of the box), separate domains—especially those with different country code TLDs (.com, .co.uk, .de) or entirely different domain names—require manual configuration.

The Root Causes (Why This Happens)

Cross-domain tracking failures stem from several technical layers:

1. Missing GA4 Admin Configuration

The most common cause: domains aren't listed in the GA4 data stream settings. GA4 uses the _gl linker parameter to pass client ID and session ID between domains. Without explicitly configuring which domains should share this data, GA4 won't append the _gl parameter to outbound links, and each domain operates in isolation.

2. GTM Misconfiguration

If you're using Google Tag Manager instead of native gtag.js, cross-domain tracking requires additional setup. You must:

  • Add the linker parameter to your GA4 Configuration tag's Fields to Set section

  • List all target domains in the linker configuration (e.g., {"domains": ["example.com", "shop.example.co.uk"]})

  • Ensure your GA4 Configuration tag fires on all pages across all domains

Missing any of these steps breaks the linker parameter chain.

3. JavaScript Execution Order Issues

The _gl parameter is added when a user clicks a link. The click event must bubble up to the document level for GA4 to intercept it and decorate the URL. If your site uses:

  • JavaScript redirects (e.g., window.location.href = ...)

  • Immediate form submissions without user interaction

  • SPA (Single Page Application) navigation that bypasses standard link clicks

...the linker parameter may never be appended. GA4's automatic link decoration only works for standard <a> tag clicks.

4. Cookie Domain Mismatches

If you've manually set a custom cookie_domain parameter in your GA4 tag configuration, it can interfere with cross-domain tracking. GA4 expects cookies to be scoped to the current domain. Overriding this with a broad domain setting (e.g., .example.com) can cause the linker parameter to be ignored.

5. Third-Party Payment Processors and Checkout Flows

Many businesses use external platforms (Stripe, PayPal, Shopify Checkout) that redirect users to a separate domain. Unless you:

  • Configure cross-domain tracking for these external domains

  • Add them to your referral exclusion list (if they redirect back)

  • Ensure the payment processor preserves URL parameters

...users will appear as new referral traffic when they return from checkout.

The "So What?" (Business Impact)

Broken cross-domain tracking has cascading consequences:

Attribution Fragmentation: Your marketing reports show inflated session counts and underreported conversion paths. A user who clicks a Google Ad, visits your main site, then completes a purchase on your checkout domain appears as:

  • Session 1: Google / CPC (main site visit)

  • Session 2: Referral / yourcompany.com (checkout domain visit)

The conversion gets attributed to "Referral" instead of "Google / CPC," destroying your ROAS calculations and making paid campaigns appear less effective than they are.

Inflated User and Session Metrics: Each domain transition creates a duplicate user. Your "Users" metric becomes meaningless. A single customer journey across three domains registers as three separate users, making engagement metrics unreliable.

Broken Funnel Analysis: If your funnel spans multiple domains (e.g., landing page → product page → external checkout), you can't accurately measure drop-off rates. Users "disappear" at domain boundaries, making it impossible to optimize conversion paths.

Self-Referral Pollution: When users return from the secondary domain to your main site, your own domain appears as a referral source. Traffic reports become cluttered with self-referrals, obscuring actual external traffic sources.

The Investigation (How to Debug)

You can manually verify cross-domain tracking issues without Watson:

Method 1: Check for the _gl Parameter

  1. Open your website in an incognito browser window

  2. Navigate to a page with a link to your secondary domain

  3. Right-click the link and select Inspect (or open Developer Tools → Network tab)

  4. Click the link and watch the URL in the browser address bar

  5. Look for ?_gl=1* in the destination URL

If the _gl parameter is missing, cross-domain tracking is not working.

Method 2: Traffic Acquisition Report Analysis

  1. In GA4, go to Reports → Acquisition → Traffic Acquisition

  2. Add a secondary dimension: Session source / medium

  3. Filter for sessions where the source is your own domain name

If you see your own domain as a referral source, sessions are breaking at domain boundaries.

Method 3: DebugView Real-Time Testing

  1. Enable Debug Mode (install the Google Analytics Debugger Chrome extension or add ?debug_mode=true to your URL)

  2. Navigate to Admin → DebugView in GA4

  3. Click through your cross-domain user flow

  4. Watch for session_start events firing when you transition between domains

If a new session_start fires on the secondary domain, the session is breaking.

Method 4: User Explorer Path Analysis

  1. Go to Reports → User → User Explorer (or Explore → User Explorer)

  2. Select a user who visited multiple domains

  3. Review the Event timeline

If you see multiple first_visit or session_start events with different referrers for what should be one journey, tracking is broken.

The Solution (How to Fix)

Option A: Native GA4 Configuration (Recommended)

This method works if you're using the native GA4 tag (gtag.js) or the Google tag directly on your site.

Step 1: Configure Domains in GA4 Admin

  1. Log in to Google Analytics and select your GA4 property

  2. Navigate to Admin (bottom left)

  3. Under Data collection and modification, click Data Streams

  4. Select your web data stream

  5. Scroll down and click Configure tag settings

  6. In the Settings section, click Configure your domains

  7. Click Add condition and select domain contains

  8. Enter each domain you want to track (e.g., example.com, shop.example.co.uk, checkout.example.com)

    • Do not include http:// or https://

    • Do not include paths (e.g., /checkout)

    • Use the root domain only

  9. Click Save

Step 2: Verify Referral Exclusions (Usually Automatic)

GA4 automatically adds configured cross-domain domains to the referral exclusion list. However, if you're seeing self-referrals:

  1. In the same Configure tag settings screen, click Show more

  2. Click List unwanted referrals

  3. Confirm your secondary domains are listed

  4. If not, manually add them

Option B: Google Tag Manager Configuration

If you're using GTM to deploy GA4, follow these steps:

Step 1: Configure GA4 Admin (Same as Above)

Complete the GA4 Admin configuration in Option A first.

Step 2: Update Your GA4 Configuration Tag in GTM

  1. Open Google Tag Manager and select your container

  2. Navigate to Tags and open your GA4 Configuration Tag

  3. Expand Fields to Set

  4. Add a new field:

    • Field Name: linker

    • Value: {"domains":["example.com","shop.example.co.uk"]}

    • Replace with your actual domains (comma-separated, in quotes)

  5. Save the tag

  6. Click Preview to test in debug mode

  7. Navigate between domains and confirm the _gl parameter appears in URLs

  8. Submit and Publish your container

Step 3: Ensure Tag Fires on All Domains

  1. Verify your GA4 Configuration tag has a trigger set to All Pages

  2. Confirm GTM is installed on every domain you're tracking

  3. Test that the tag fires on both domains using Preview mode

Case Closed

Manually investigating cross-domain tracking issues requires checking multiple reports, testing URL parameters, and validating session continuity across domains—a process that can take 30+ minutes per property. The Watson Analytics Detective dashboard spots this Info-level check instantly, alongside 60+ other data quality issues. Watson automatically flags whether your cross-domain configuration is complete, showing you exactly which domains are tracked and which user journeys are breaking.

Stop hunting for attribution gaps manually. Let Watson audit your GA4 setup in seconds :www.analyticsdetectives.com/watson

Ali Izadi
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