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.

From COD HQ, head to the Modern Warfare III section and select the Campaign tile. Even though the game has already launched, we don't want to spoil anything about it. Follow the prompts when starting the Campaign to choose your difficulty setting and then get right to it.