Today we’re releasing a feature to help you discover if your site serves undesirable "soft” or “crypto” 404s. A "soft 404" occurs when a webserver responds with a 200 OK HTTP response code for a page that doesn’t exist rather than the appropriate 404 Not Found. Soft 404s can limit a site’s crawl coverage by search engines because these duplicate URLs may be crawled instead of pages with unique content.
The web is infinite, but the time search engines spend crawling your site is limited. Properly reporting non-existent pages with a 404 or 410 response code can improve the crawl coverage of your site’s best content. Additionally, soft 404s can potentially be confusing for your site’s visitors.
You can find the new soft 404s reporting feature under the Crawl errors section in Webmaster Tools.
Here’s a list of steps to correct soft 404s to help both Google and your users:
1. Check whether you have soft 404s listed in Webmaster Tools
2. For the soft 404s, determine whether the URL:
a. Contains the correct content and properly returns a 200 response (not actually a soft 404)
b. Should 301 redirect to a more accurate URL
c. Doesn’t exist and should return a 404 or 410 response
3. Confirm that you’ve configured the proper HTTP Response by using Fetch as Googlebot in Webmaster Tools
4. If you now return 404s, you may want to customize your 404 page to aid your users.
We hope that you’re now better enabled to find and correct soft 404s on your site.