404 errors are the silent killers of user experience. Every dead page is a visitor who bounces, and enough of them signals neglect to search engines.

The most common causes: deleted pages without redirects, mistyped internal links, and external sites linking to URLs that no longer exist.

A proper 404 error checker doesn't just find the broken page — it shows you WHICH pages link to it, so you can fix the source instead of playing whack-a-mole.

Scan any site below to find every 404 in seconds, then 301-redirect the important ones to keep your link equity.