1. Dead Pages with LinksWhat it is: Sites regularly go through changes but along the way old URLs get left behind. This is normal but if the old URLs were often linked to a huge amount of your ability to rank in Google is pointed at dead end pages where those links can’t pass their value.
How you check it: Go to Open Site Explorer and search for your domain. Select the top pages report and look for pages that have external links and a 400 level HTTP status.
How you fix it: Create a full top page report then export the list to a CSV.