Occasionally, when you are using the Page Experience module you will see a red warning triangle next to a URL and a row of exclamation marks indicating that the Lighthouse API was unable to report on the page's Core Web Vitals.

This indicates that there was a problem when the Lighthouse API tried to render the page. This can be due to a number of reasons - some of them indicating some serious SEO issues that may well be worthy of further investigation.

If you hover over the red warning triangle then you will see an error message with some technical details of what failed. e.g.
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"SSL error encountered",
"The page did not paint any content",
"DNS servers could not resolve the provided domain",
"Page loading failed",
"Unknown error"

For the non-technically minded these are some of the most common issues we have observed whilst running Lighthouse API checks across thousands of URLs.

How to troubleshoot Lighthouse API errors

Step 1 - Before going any further please do a manual check of the URL in your browser.

Sometimes, a simple typo or getting the wrong http:// or https:// protocol may cause an issue (even when we follow redirects). 
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This may not help you spot other less obvious issues, e.g. you may not get a Captcha if you are using a recognised browser and user-agent, but the site may return a Captcha to the Lighthouse API. So, please check this list of other common reasons for failure that we have spotted.
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Step 2 - Review our list of other common reasons for Lighthouse API failure

• The page contains major JavaScript errors that prevent Lighthouse from loading.

• The page returns a Captcha to the Lighthouse API.

• The website is geo-blocking our Lighthouse requests (we check from France, the UK and the United States).

• The page loads so slowly that the process reaches the Lighthouse API time-out.

• The page has problems with its SSL certificate and these warning prevent the page loading automatically

• The server returns an error such as a 5XX server error, a 404 (page not found) or 403 (not authorised error)

• The page redirects too many times or redirects to a parked domain or domain suspended page

Please note, we can get only the output from the Lighthouse application that uses Chrome in the background, so some browser errors such as DNS_PROBE_FINISHED_NXDOMAIN or ERR_CONNECTION_TIMED_OUT may not necessarily be reported, but if you manually check the URL you should be able to spot this in your browser.