From Fedora Project Wiki

Yum not working?

Summary

Recently, http://mirrors.fedoraproject.org/mirrorlist, the script which returns a list of mirrors based on release, arch, and geographic location, has been giving 5** errors. Since the default yum configuration relies on this script to get mirror lists, this problem pretty much breaks yum. This problem is currently being worked on, but until it is fixed, you might want to try the quick fix below.

Quick fix

1. Go to http://mirrors.fedoraproject.org/ and choose a mirror list (one of the txt files) based on your repo, arch, release, and country. 2. Edit the repo files in /etc/yum.repos.d to use the mirror list based on the choice above.

Example: I'm using FC6 in the US on i386, so my /etc/yum.repos.d/fedora-core.repo would have the following mirrorlist line:

mirrorlist=http://mirrors.fedoraproject.org/core-6-US-i386.txt