Source: http://www.searchengineguide.com/jayson-demers/can-404-errors-hurt-your-seo-rankings.php
Most probably, you have already encountered an error 404 at least once. Not only when visiting other websites, maybe even on your own site. A webpage that returns error 404 means that the specific page does not exist or otherwise, cannot be accessed.
Other websites have a default 404 error page which informs visitors that the domain is live, however, the requested page does not exist. While some webmasters let these catch-all pages handle everything, some also spend effort in eliminating 404 errors.
Is SEO rankings really affected by 404 errors?
Error 404 occurs when you:
- Forgot to update your sitemap or external links when deleting or renaming a page.
- Changed your page’s URL without updating your sitemap or external links.
- A user/visitor mistyped a live page resulting to error 404.
Impact in Rankings:
Actually, error 404 impacting your SEO rankings is nothing but a myth. There is nothing to worry about, because there is no inherent penalty from Google for having returned a 404 error for a page on your website. It has nothing to do with the way Google sees the other pages of your website nor does it incur any type of penalty if too many 404 errors exist.
Then what effects does it have?
Although it does not directly affect your rankings, it doesn’t mean you can ignore them, and this is because of the overall user experience. They can detract from your overall user experience, thus gaining negative effect on your SEO goals.
It can also leave an impression on users and prevent them from returning especially when they are expecting a page of content when clicking a link or visiting from a bookmark. So if a specific page is important to your business, and it returns a 404 error, it may drop in rank, hence it is important to correct 404 errors to improve user experience, and therefore, increase your traffic and visibility.
Fixing 404 errors
- Revert the URL back to its original form as if nothing happened (in case you removed or renamed the page)
- Applying 301 redirects
- Ensure that your sitemap, redirects and pages are all up-to-date
Generally, 404 errors won’t directly harm your rankings but aren’t healthy for your users or your overall marketing strategy. It is recommended that you check your site for page errors and perform the necessary changes to keep it up-to-date.