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Test Errors

This article explains common test errors. Feel free to contact support if you need help with an error you see on DebugBear.

DebugBear built on top of Lighthouse, so many of these errors will match Lighthouse ones.

Lighthouse test error

Chrome prevented page load with an interstitial

Chrome was unable to load the website. Check the test screenshot to see the exact error message.


Resolving the domain name to an IP address failed. Make sure you're not testing a website that's only available on your internal company network.

Protocol timeout

No response was received when talking to Chrome. This can happen for different reasons.

For example, this happens if no response was received to the document request. The error does not indicate an HTTP error response – rather it indicates the absence of a response.

Protocol timeouts can also occur when specific data gathering steps take a long time. Some Lighthouse audits run JavaScript code on the page, for example to gather data on all links. If your page contains 10,000 links then the page main thread might end up blocked and the command will time out.

Couldn't reliably load the page you requested

The request for the HTML document failed with an error code. For example, the website server might have response with a 404 Not Found or 403 Forbidden response.

Lighthouse doesn't currently support testing 404 or 500 pages.

If you get an 403 error, make sure that your server isn't blocking the DebugBear test agent. Set up static IP addresses and allowlist them in your server configuration.

Sometimes you will see test result data despite this test error being shown. That usually means that an error code was returned for some but not all document requests. When running a Lighthouse test the page is loaded multiple times (multiple "passes").

No First Contentful Paint

The Lighthouse NO_FCP error indicates that no content like text or images was rendered.

Some reasons for this could be:

  • the page took more than 15 seconds to start rendering – this often happens when running tests on mobile
  • the page renders a JavaScript app and the application crashed

No Speed Index

Speed Index looks at rendering snapshots and evaluates how much of the page has rendered. If no content is rendered a SPEEDINDEX_OF_ZERO is returned.

This can also happen if content is initially rendered but then disappears.

The Speed Index metric is used to calculate the Lighthouse Performance score, so without it the Performance score will also be missing.

Chrome snapshot limit reached

Chrome can only capture up to 450 filmstrip snapshots. For most sites this is plenty, but if your pages takes more than 10 seconds to load and shows a video or animation throughout this time then Chrome will eventually stop collecting snapshots.

If you need a full filmstrip you can inject a script that disables the animation.