One often overlooked free webmaster tool that Google provides for free is a link diagnostic tool that lets you analyze all the links on your website to determine what web pages link to your 404 error pages. You can use this tool to identify these links and help your search engine optimization efforts. Also by using this tool you can get information needed to contact the owners of those websites to have them point their links to the right pages. Why would you do this? Because the search engines are likely not counting inbound links that point to your 404 pages.
This often happens when webmasters give a certain page a drastic redesign and change the name of the file in the process. Sometimes, changing the file name is inadvertent and the webmaster leaves out a character or adds one, or he may forget the exact name he gave the last file and name the new file something similar but different. Thinking that he’s written over the old file, he uploads the file name after having deleted the old one. Anyone who visits the old file will get a 404 error page.
If you know the name of the old file that is catching these 404 error page links then you can simply redirect that page to your new page and that will fix it. It will at least divert traffic to your new page. But it won’t really help you gain inbound links. What you’ll need to do is contact the owners of websites linking to that old page and give them the URL of your new page.
To find out which pages are linking to your 404 error pages, sign into Webmaster Central and click on the website you want to check on. Click on “Diagnostics” then click on “Web crawl”. Click on “Not found” and you’ll see a list of links that the search engines are not crawling. You should see a series of links that say something like “2 pages” or “14 pages”. Those are the number of pages linking to that 404 error page. Click the hyperlink and you’ll see the exact pages linking to the error page. They’ll either be internal pages on your site or offsite pages and if the latter then you’re losing valuable inbound link juice. That’s when you should visit those websites and send an e-mail to the owner to let them know that there is another page they can link to.