There are different types of 404 errors on a webpage:
- 404 error from an image URL located in a IMG SRC tag
- 404 error from a URL on the same domain located in a A HREF tag
- 404 error from a URL on a different domain located in a A HREF tag
Furthermore, a webpage may not not exist anymore and all its incoming links may have been deleted too. However, this particular URL may still exist in the Google index.
Unfortunately, Google isn’t clear about 404 errors. We don’t know if those 404 errors may cause a traffic drop.
404 errors cause Google juice leakages
However, all the SEO consultants do agree about one thing: 404 errors are google juice leakages. The Internet is a very competitive environment and it is important to keep as much Google juice as possible for the web pages. On the original technical paper from Google, it states that if we have 10 links on a page, the google juice is divided by 10. If 2 of those internal links are 404 errors, it means that 20% of the google juice is lost. Removing the 2 links that return a 404 error would benefit the other links because the google juice would be divided by 8.
Below you can see a chart of the original Google juice algorithm as designed in 1997 by Larry Page. If a link does not work in the said chart, it means that the google juice is lost for the said link.
My opinion about 404 errors on webpages
How many times do we visit a web pages with 404 errors on it after clicking on a google search result? NEVER
However, those pages exist, there are the forum threads. Have you ever found a forum thread from google search results where images returned a 404 errors and where several internal links returned a 404 error?
It’s been a while that I have been found this type of webpages from a google search result. Consequently, I think web pages with several internal 404 errors in A HREF and IMG SRC are downrated by google.