Clear the Cache of Your Console. Another common reason for the Call of Duty Modern Warfare Multiplayer not working error is the corrupted cache data. In this case, you can try clearing the cache files on your PC or console to fix the issue.
Your bread and butter: Multiplayer is the foundation of Call of Duty, and it's no different in Modern Warfare III. If you're new here, get right into it by jumping around the different Playlists.