Debug Tag Monitor errors in Tag Manger

These guidelines are constantly updated based on best practices we find along the way. Do you have any specific findings? Please help us get these docs as complete as possible.

General Debugging Tips

Before diving into specific debugging techniques, perform these general checks to understand the error context.

• Check which container version gave the error. Google Tag Manager has quite a thorough cache and therefore some older container versions might still give errors as these versions are quite persistent.
• Check when the error started; did a change take place in tracking around that time?
• Analyse on which page the error was encountered. Is there a specific page or page type that triggers the error?
• Check if the tag that gives the error is paused. Tip: always remove the trigger from tags that are paused. Even better, delete these tags altogether. They can always be found back via version history and won't clutter your tag configuration or trigger unnecessary processing.
• Use both Tag Manager's preview mode and the browser developer console for debugging:
• Try to relocate a page where the tag was marked as 'failed'. Open Tag Manager in preview mode and navigate to this page. If the tag still fails, use both Tag Manager's preview mode and the browser developer console for debugging:
• The ‘Console’ tab (found in the GTM preview mode) often shows errors related to triggers, tag conditions, or consent.
• The browser console (F12 → Console tab) can reveal JavaScript errors from custom HTML tags or third-party scripts, which often explain why a tag fails to fire.

Debugging errors in Google Tag Manager client-side

Below some tips and tricks on debugging errors in your Google Tag Manager client-side environment. In most cases debugging errors in your client-side container is trial and error, but these tips might help you along the way.

General client-side tips

• Be aware of tag sequencing. If a tag depends on another tag (e.g. consent or pixel config), make sure to configure tag sequencing correctly to avoid firing errors.
• Double-check consent settings. For tags relying on Consent Mode (like Google Ads or LinkedIn), ensure that the appropriate consent types are granted before the tag attempts to fire. Otherwise, they silently fail.

Microsoft Ads

To resolve Microsoft Ads tracking issues:

• Ensure the 'UET config' tag is installed and fires on every page
• Make sure to add the right variable for the parameter 'UETQ variable ID'. This is for example 'uetq'.
• Make sure Microsoft Consent Mode has been configured correctly.

LinkedIn Insight

• The LinkedIn tag gives an error when you try to fire the event before a consent update has been sent. Try and send it a bit later.

Debugging errors in Google Tag Manager server-side

Below some tips and tricks on debugging errors in your Google Tag Manager server-side environment.

GTM server-side container gives more insights on the errors than the client-side container. Do you encounter a high percentage of failed tags (+75%)? Make sure to open the preview mode and look into the tab 'Console'. The chances are quite high more information on the specific error can be found here.

General Server-Side Tips

• Use the trigger type 'Custom' as little as possible. This one is too generic and therefore causes quite often a lot of triggers.
• Do you want to fire on all events? It is better to use trigger type 'Custom Event' with a wildcard.
• Check if the client handling the event is implemented correctly.
• Inspect incoming event payloads. Use the Event Data tab in GTM server-side preview to check what data is actually received. Misformatted or missing parameters are a common cause of failed conversions or missing events.
• Verify client mappings. If events are not handled correctly, double-check that the correct GTM Client (e.g., GA4, Universal Analytics, HTTP) is active and properly matching incoming requests.

Google Ads

Conversion Linker failing? Make sure the region specific settings are added in your server-side instance (Cloud Run for example).

Tag incorrectly marked as ‘failed’? In some cases, the Google Ads Conversion tag is marked as failed, even when conversions are tracked correctly in Google Ads. This is a known false error due to cookie prefix mismatches.

👉 Read how to fix this issue

Facebook Conversion API

• Facebook is very strict on all event parameters. When a Facebook tag fails, this is mostly caused by an event parameter not having the right value.
• Using a variable to fill in the value of the event parameter? Make sure to always add a default value to send.
• Check the trigger added to the tag. Is the trigger specific enough? It might be the trigger fires on events where you don't want it to fire, so it can't fill in all parameters and throws an error. Make sure to also check our general tips on setting up triggers in the server-side environment.

TikTok Conversion API

• Facebook is very strict on all event parameters. When a Facebook tag fails, this is mostly caused by an event parameter not having the right value.
• Using a variable to fill in the value of the event parameter? Make sure to always add a default value to send.
• Check the trigger added to the tag. Is the trigger specific enough? It might be the trigger fires on events where you don't want it to fire, so it can't fill in all parameters and throws an error. Make sure to also check our general tips on setting up triggers in the server-side environment.

Criteo

• Make sure to check if all parameters are configured correctly.
• Using a variable to fill in the value of the event parameter? Make sure to always add a default value to send.

Additional Resources

• Offload data to BigQuery

Need help with specific debugging scenarios? Contact our support team for personalized assistance.