Recently, I’ve encountered an unusual and frustrating issue where my Google Ads conversion tag suddenly started failing. The Google Ads conversion tracking template provided within Google Tag Manager is something of a “black box,” so diagnosing the root cause hasn’t been straightforward.

What’s Happening?
At first, I suspected a consent issue. Consent settings can often cause conversion tags to fail if they’re not properly configured or set before the conversion events fire. Adjusting these settings typically solves such problems: ensure ad_storage, ad_personalization and ad_user_data consent is granted, use a Consent Initialization tag in GTM, and sequence tags so the global site tag and consent states are established before conversion tags run.

However, in my case, the consent setup was verified and correct. Yet the tag continued to fail at certain times. After some painstaking debugging, I narrowed the cause down to a particular cookie: _gcl_au.

The Role of _gcl_au
The _gcl_au cookie is associated with the Conversion Linker tag in Google Tag Manager. This cookie helps attribute conversions back to clicks and is crucial for proper Google Ads measurement. Strangely, when I was logged out of my site, everything worked perfectly. When logged in, the conversion tag would fail—coinciding with the presence of an unusually long _gcl_au cookie.

The length and format of the _gcl_au cookie were changing between logged-in and logged-out states. For example:

• Logged Out _gcl_au: 1.1.749169968.1734352994
• Logged In _gcl_au: 1.1.1076316173.1734348272.1690549490.1734352745.1734352753

Update, the issue is with the advanced conversions. So when some goes to login, the enhanced conversion adds something to the cookie. This is causing it to fail.

This discrepancy began appearing at the end of October, right around when I made some minor campaign adjustments. Initially, I assumed the performance dip was due to the algorithm “relearning” and that it would bounce back. But it never did, even after reverting to the previous setup. I brought in another expert who confirmed that my plan was solid—on paper, everything should have worked.

A Clue From Brand Campaigns
The real “aha” moment came from examining a brand campaign. Brand campaigns are usually stable and have a steady conversion rate. Yet at the end of October, even the brand campaign’s reported conversion rate took a nosedive in Google Ads. Meanwhile, my GA4 reports showed no such drop—conversion rates remained steady there.

This discrepancy suggests the issue resides exclusively on the Google Ads side of measurement. The failure of the Ads conversion tag due to _gcl_au cookie issues explains the sudden drop in recorded conversions. Conversions were still happening—GA4 proved that—but Ads wasn’t capturing them properly.

Nightmare for clients and agencies

This type of tracking issue is a nightmare scenario for marketing agencies. Clients often see a drop in reported conversions and immediately assume the agency's strategy isn't working. I've been in situations where clients were ready to terminate the contract, convinced the agency was at fault.

Luckily, because of the untouched brand campaign, I was able to pinpoint the technical glitch and demonstrate that conversions were actually happening, just not being tracked correctly. Many agencies aren't so fortunate and get fired for problems entirely outside their control. These situations underscore the critical need for robust tracking setups and a thorough understanding of how different platforms interact.

Whats worse is the next agency won't be able to fix either as its a technical problem, rather than anything wrong with the campaigns.

If You’re Experiencing This Issue…
If this scenario sounds familiar—conversions failing intermittently, _gcl_au behaving strangely, performance metrics declining without a logical explanation—your setup may have a similar cookie or tag sequencing problem. Consent mode needs to be properly initialized, cookies must be set correctly (often requiring HTTPS and updated cookie flags), and tags should fire in the correct order to ensure the conversion tag doesn’t fail.

I’m currently working through this problem on this client's site. If you’re facing a similar challenge, I’d be happy to take a quick look for free. Solving your issue might help me solve mine, and together we can ensure that the data we rely on from Google Ads is accurate and consistent.

Fall back Solution

The fallback solution would be to capture the Google ClickIDs and then use offline conversion tracking. Ideally, I don't want to do that because I'd have to change all my enhanced conversions I've set up, and I'd rather just fix the existing thing rather than starting again with another tracking method.

Other people with the same problem.

I have found other people with the same problem.

On reddit https://www.reddit.com/r/GoogleTagManager/comments/1hbv583/firing_status_failed_although_the_trigger_is/

On the Google Support forum

https://support.google.com/tagmanager/thread/311624206/help-needed-google-ads-conversion-tag-firing-failed?hl=en

Update

You can get rid of the error by changing the cookie settings in the conversion linker.

There's something to do with something else setting the cookie and overriding the correct one so by changing the cookie the conversion linker sets will help. In my case there were other things wrong so it's not totally solved but for a lot of other people that will solve it.

Other Warning.

This Google tag should have been loaded before sending any events. To resolve this, add a Google tag for AW-814809422 to your container in addition to any conversion linker, Google Ads, or Floodlight tags. Make sure that the Google tag is set to fire on the ‘Initialisation – All pages' trigger.

The warning itself does not cause any major issues with your data collection or tracking functionality. If you're using dedicated Google Ads conversion tracking tags, remarketing tags, and the conversion linker tag, your tracking will continue to work properly despite this warning.

However, by not implementing the recommended Google tag setup, you miss out on some benefits:

1. You won't be able to create remarketing lists in your Google Ads account using the tag
2. You won't be able to create page-based conversion goals

To resolve this warning, you can:

1. Add a Google tag with your Google Ads tag ID (AW-XXXXXX) to your container
2. Configure it to fire on the “Initialization – All Pages” trigger

If you choose to ignore the warning, your existing Google Ads conversion tracking and remarketing implementations will continue to function normally[3]. This is more of an optimization recommendation from Google rather than a critical error that needs immediate attention.

The funny thing is that when I actually implemented this it broke it again so I've not fixed that error. I have my issue fixed in the debugger but waiting to see if it fixes it in the real world.

Ben

Ben has a BEng (Hons) in Computer Science and 20 years of experience in online marketing, specialising in SEO, lead generation and affiliate marketing. After spending over a decade as an igaming affiliate, he has decided to concentrate on GA4 training and SEO Audits.